diff --git a/packages/core/src/submodules/protocols/cbor/CborCodec.spec.ts b/packages/core/src/submodules/protocols/cbor/CborCodec.spec.ts
new file mode 100644
index 000000000000..c5438929c935
--- /dev/null
+++ b/packages/core/src/submodules/protocols/cbor/CborCodec.spec.ts
@@ -0,0 +1,72 @@
+import { CborCodec } from "@smithy/core/cbor";
+import { describe, test as it } from "vitest";
+
+import { createNestingWidget, nestingWidget } from "../test-schema.spec";
+
+describe("performance baseline indicator", () => {
+  const codec = new CborCodec();
+  const serializer = codec.createSerializer();
+  const deserializer = codec.createDeserializer();
+
+  it("should serialize objects", () => {
+    const timings: string[] = [];
+    const objects = [];
+
+    // warmup
+    for (let i = 0; i < 13; ++i) {
+      const o = createNestingWidget(2 ** i);
+      objects.push(o);
+      serializer.write(nestingWidget, o);
+      serializer.flush();
+    }
+
+    for (let i = 0; i < objects.length; ++i) {
+      const o = objects[i];
+
+      const A = performance.now();
+      serializer.write(nestingWidget, o);
+      const serialization = serializer.flush();
+      const B = performance.now();
+
+      timings.push(
+        `${B - A} (byte length = ${serialization.byteLength}, ${serialization.byteLength / 1024 / (B - A)} kb/ms)`
+      );
+    }
+
+    /**
+     * No assertion here.
+     * In the initial dual-pass implementation,
+     * par time is 0 to 23ms for up to 381014 bytes of CBOR. Up to 15 kb/ms. (kuhe's computer)
+     */
+    console.log("CborShapeSerializer performance timings", timings);
+  });
+
+  it("should deserialize bytes", async () => {
+    const timings: string[] = [];
+    const strings = [];
+
+    // warmup
+    for (let i = 0; i < 12; ++i) {
+      const o = createNestingWidget(2 ** i);
+      serializer.write(nestingWidget, o);
+      const json = serializer.flush();
+      strings.push(json);
+      await deserializer.read(nestingWidget, json);
+    }
+
+    for (const s of strings) {
+      const A = performance.now();
+      await deserializer.read(nestingWidget, s);
+      const B = performance.now();
+
+      timings.push(`${B - A} (byte length = ${s.byteLength}, ${s.byteLength / 1024 / (B - A)} kb/ms)`);
+    }
+
+    /**
+     * No assertion here.
+     * In the initial dual-pass implementation,
+     * par time is 0 to 9ms for up to 190550 bytes of CBOR. Up to 23 kb/ms. (kuhe's computer)
+     */
+    console.log("CborShapeDeserializer performance timings", timings);
+  });
+}, 30_000);
diff --git a/packages/core/src/submodules/protocols/json/JsonShapeDeserializer.spec.ts b/packages/core/src/submodules/protocols/json/JsonShapeDeserializer.spec.ts
index 231450c63588..56c639bf93ae 100644
--- a/packages/core/src/submodules/protocols/json/JsonShapeDeserializer.spec.ts
+++ b/packages/core/src/submodules/protocols/json/JsonShapeDeserializer.spec.ts
@@ -2,8 +2,9 @@ import { NumericValue } from "@smithy/core/serde";
 import type { TimestampEpochSecondsSchema } from "@smithy/types";
 import { describe, expect, test as it } from "vitest";
 
-import { widget } from "../test-schema.spec";
+import { createNestingWidget, nestingWidget, widget } from "../test-schema.spec";
 import { JsonShapeDeserializer } from "./JsonShapeDeserializer";
+import { JsonShapeSerializer } from "./JsonShapeSerializer";
 
 describe(JsonShapeDeserializer.name, () => {
   let contextSourceAvailable = false;
@@ -153,4 +154,45 @@ describe(JsonShapeDeserializer.name, () => {
     expect(await deserializer.read(widget, JSON.stringify({ scalar: "-Infinity" }))).toEqual({ scalar: -Infinity });
     expect(await deserializer.read(widget, JSON.stringify({ scalar: "NaN" }))).toEqual({ scalar: NaN });
   });
+
+  describe("performance baseline indicator", () => {
+    const serializer = new JsonShapeSerializer({
+      jsonName: true,
+      timestampFormat: { default: 7 satisfies TimestampEpochSecondsSchema, useTrait: true },
+    });
+    serializer.setSerdeContext({
+      base64Encoder: (input: Uint8Array) => {
+        return Buffer.from(input).toString("base64");
+      },
+    } as any);
+
+    it("should deserialize JSON strings", async () => {
+      const timings: string[] = [];
+      const strings = [];
+
+      // warmup
+      for (let i = 0; i < 12; ++i) {
+        const o = createNestingWidget(2 ** i);
+        serializer.write(nestingWidget, o);
+        const json = serializer.flush();
+        strings.push(json);
+        await deserializer.read(nestingWidget, json);
+      }
+
+      for (const s of strings) {
+        const A = performance.now();
+        await deserializer.read(nestingWidget, s);
+        const B = performance.now();
+
+        timings.push(`${B - A} (JSON length = ${s.length}, ${s.length / 1024 / (B - A)} kb/ms)`);
+      }
+
+      /**
+       * No assertion here.
+       * In the initial dual-pass implementation,
+       * par time is 0 to 25ms for up to 288899 chars of JSON. Up to 13 kb/ms. (kuhe's computer)
+       */
+      console.log("JsonShapeDeserializer performance timings", timings);
+    });
+  }, 30_000);
 });
diff --git a/packages/core/src/submodules/protocols/json/JsonShapeSerializer.spec.ts b/packages/core/src/submodules/protocols/json/JsonShapeSerializer.spec.ts
index 0e6423381605..921215b003d6 100644
--- a/packages/core/src/submodules/protocols/json/JsonShapeSerializer.spec.ts
+++ b/packages/core/src/submodules/protocols/json/JsonShapeSerializer.spec.ts
@@ -2,31 +2,75 @@ import { NumericValue } from "@smithy/core/serde";
 import type { TimestampEpochSecondsSchema } from "@smithy/types";
 import { describe, expect, test as it } from "vitest";
 
-import { widget } from "../test-schema.spec";
+import { createNestingWidget, nestingWidget, widget } from "../test-schema.spec";
+import { SinglePassJsonShapeSerializer } from "./experimental/SinglePassJsonShapeSerializer";
 import { JsonShapeSerializer } from "./JsonShapeSerializer";
 
 describe(JsonShapeSerializer.name, () => {
-  it("serializes data to JSON", async () => {
-    const serializer = new JsonShapeSerializer({
-      jsonName: true,
-      timestampFormat: { default: 7 satisfies TimestampEpochSecondsSchema, useTrait: true },
-    });
-    serializer.setSerdeContext({
-      base64Encoder: (input: Uint8Array) => {
-        return Buffer.from(input).toString("base64");
-      },
-    } as any);
+  const serializer1 = new JsonShapeSerializer({
+    jsonName: true,
+    timestampFormat: { default: 7 satisfies TimestampEpochSecondsSchema, useTrait: true },
+  });
+
+  const serializer2 = new SinglePassJsonShapeSerializer({
+    jsonName: true,
+    timestampFormat: { default: 7 satisfies TimestampEpochSecondsSchema, useTrait: true },
+  });
 
+  it("serializes data to JSON", async () => {
     const data = {
       timestamp: new Date(0),
       bigint: 10000000000000000000000054321n,
       bigdecimal: new NumericValue("0.10000000000000000000000054321", "bigDecimal"),
       blob: new Uint8Array([0, 0, 0, 1]),
     };
-    serializer.write(widget, data);
-    const serialization = serializer.flush();
+    serializer1.write(widget, data);
+    const serialization = serializer1.flush();
     expect(serialization).toEqual(
       `{"blob":"AAAAAQ==","timestamp":0,"bigint":10000000000000000000000054321,"bigdecimal":0.10000000000000000000000054321}`
     );
   });
+
+  describe("performance baseline indicator", () => {
+    for (const serializer of [serializer1, serializer2]) {
+      it("should serialize objects", () => {
+        const timings: string[] = [];
+        const objects = [];
+
+        // warmup
+        for (let i = 0; i < 12; ++i) {
+          const o = createNestingWidget(2 ** i);
+          objects.push(o);
+          serializer.write(nestingWidget, o);
+          serializer.write(nestingWidget, o);
+          serializer.write(nestingWidget, o);
+          serializer.write(nestingWidget, o);
+          serializer.flush();
+        }
+
+        for (let i = 0; i < objects.length; ++i) {
+          const o = objects[i];
+
+          const A = performance.now();
+          serializer.write(nestingWidget, o);
+          const serialization = serializer.flush();
+          const B = performance.now();
+
+          timings.push(
+            `${B - A} (JSON length = ${serialization.length}, ${serialization.length / 1024 / (B - A)} kb/ms)`
+          );
+        }
+
+        /**
+         * No assertion here.
+         * In the initial dual-pass implementation,
+         * par time is 0 to 30ms for up to 288899 chars of JSON. Up to 11 kb/ms. (kuhe's computer)
+         *
+         * In the single-pass implementation using string buildup,
+         * par time is 0 to 51ms for up to 288899 chars of JSON. Up to 13 kb/ms. (kuhe's computer)
+         */
+        console.log(`${serializer.constructor.name} performance timings`, timings);
+      });
+    }
+  }, 30_000);
 });
diff --git a/packages/core/src/submodules/protocols/json/JsonShapeSerializer.ts b/packages/core/src/submodules/protocols/json/JsonShapeSerializer.ts
index 9f9678a166b4..ebba45091f86 100644
--- a/packages/core/src/submodules/protocols/json/JsonShapeSerializer.ts
+++ b/packages/core/src/submodules/protocols/json/JsonShapeSerializer.ts
@@ -102,10 +102,7 @@ export class JsonShapeSerializer extends SerdeContextConfig implements ShapeSeri
       if (ns === this.rootSchema) {
         return value;
       }
-      if (!this.serdeContext?.base64Encoder) {
-        return toBase64(value);
-      }
-      return this.serdeContext?.base64Encoder(value);
+      return (this.serdeContext?.base64Encoder ?? toBase64)(value);
     }
 
     if ((ns.isTimestampSchema() || ns.isDocumentSchema()) && value instanceof Date) {
diff --git a/packages/core/src/submodules/protocols/json/experimental/SinglePassJsonShapeSerializer.ts b/packages/core/src/submodules/protocols/json/experimental/SinglePassJsonShapeSerializer.ts
new file mode 100644
index 000000000000..da2a99a2804c
--- /dev/null
+++ b/packages/core/src/submodules/protocols/json/experimental/SinglePassJsonShapeSerializer.ts
@@ -0,0 +1,168 @@
+import { determineTimestampFormat } from "@smithy/core/protocols";
+import { NormalizedSchema } from "@smithy/core/schema";
+import { dateToUtcString, generateIdempotencyToken, LazyJsonString, NumericValue } from "@smithy/core/serde";
+import type {
+  Schema,
+  ShapeSerializer,
+  TimestampDateTimeSchema,
+  TimestampEpochSecondsSchema,
+  TimestampHttpDateSchema,
+} from "@smithy/types";
+import { toBase64 } from "@smithy/util-base64";
+
+import { SerdeContextConfig } from "../../ConfigurableSerdeContext";
+import type { JsonSettings } from "../JsonCodec";
+
+/**
+ * This implementation uses single-pass JSON serialization with JS code instead of
+ * JSON.stringify.
+ *
+ * It isn't significantly faster than dual-pass ending with native JSON.stringify
+ * that I would want to use it. It seems to be barely faster in some mid-range object
+ * sizes but slower on the high end.
+ *
+ * @internal
+ */
+export class SinglePassJsonShapeSerializer extends SerdeContextConfig implements ShapeSerializer<string> {
+  private buffer: any;
+  private rootSchema: NormalizedSchema | undefined;
+
+  public constructor(public readonly settings: JsonSettings) {
+    super();
+  }
+
+  public write(schema: Schema, value: unknown): void {
+    this.rootSchema = NormalizedSchema.of(schema);
+    this.buffer = this.writeObject(this.rootSchema, value);
+  }
+
+  /**
+   * @internal
+   */
+  public writeDiscriminatedDocument(schema: Schema, value: unknown): void {
+    this.write(schema, value);
+    if (typeof this.buffer === "object") {
+      this.buffer.__type = NormalizedSchema.of(schema).getName(true);
+    }
+  }
+
+  public flush(): string {
+    this.rootSchema = undefined;
+
+    return this.buffer;
+  }
+
+  private writeObject(schema: Schema, value: unknown): string {
+    if (value == undefined) {
+      return "";
+    }
+
+    let b = "";
+    const ns = NormalizedSchema.of(schema);
+    const sparse = !!ns.getMergedTraits().sparse;
+
+    if (Array.isArray(value) && (ns.isDocumentSchema() || ns.isListSchema())) {
+      b += "[";
+      for (let i = 0; i < value.length; ++i) {
+        const item = value[i];
+        if (item != null || sparse) {
+          b += this.writeValue(ns.getValueSchema(), item);
+          b += ",";
+        }
+      }
+    } else if (ns.isStructSchema()) {
+      b += "{";
+      for (const [name, member] of ns.structIterator()) {
+        const item = (value as any)[name];
+        const targetKey = this.settings.jsonName ? member.getMergedTraits().jsonName ?? name : name;
+        const serializableValue = this.writeValue(member, item);
+        if (item != null || member.isIdempotencyToken()) {
+          b += `"${targetKey}":${serializableValue}`;
+          b += ",";
+        }
+      }
+    } else if (ns.isMapSchema() || ns.isDocumentSchema()) {
+      b += "{";
+      for (const [k, v] of Object.entries(value)) {
+        if (v != null || sparse) {
+          b += `"${k}":${this.writeValue(ns, v)}`;
+          b += ",";
+        }
+      }
+    }
+
+    if (b[b.length - 1] === ",") {
+      b = b.slice(0, -1);
+    }
+    if (b[0] === "[") {
+      b += "]";
+    }
+    if (b[0] === "{") {
+      b += "}";
+    }
+    return b;
+  }
+
+  private writeValue(schema: Schema, value: unknown): string {
+    const isObject = value !== null && typeof value === "object";
+
+    const ns = NormalizedSchema.of(schema);
+    const quote = (_: string) => `"${_}"`;
+
+    if (
+      (ns.isBlobSchema() && (value instanceof Uint8Array || typeof value === "string")) ||
+      (ns.isDocumentSchema() && value instanceof Uint8Array)
+    ) {
+      return quote((this.serdeContext?.base64Encoder ?? toBase64)(value));
+    }
+
+    if ((ns.isTimestampSchema() || ns.isDocumentSchema()) && value instanceof Date) {
+      const format = determineTimestampFormat(ns, this.settings);
+      switch (format) {
+        case 5 satisfies TimestampDateTimeSchema:
+          return quote(value.toISOString().replace(".000Z", "Z"));
+        case 6 satisfies TimestampHttpDateSchema:
+          return quote(dateToUtcString(value));
+        case 7 satisfies TimestampEpochSecondsSchema:
+          return String(value.getTime() / 1000);
+        default:
+          console.warn("Missing timestamp format, using epoch seconds", value);
+          return String(value.getTime() / 1000);
+      }
+    }
+
+    if (ns.isNumericSchema() && typeof value === "number") {
+      if (Math.abs(value) === Infinity || isNaN(value)) {
+        return quote(String(value));
+      }
+    }
+
+    if (ns.isStringSchema()) {
+      if (typeof value === "undefined" && ns.isIdempotencyToken()) {
+        return quote(generateIdempotencyToken());
+      }
+
+      if (typeof value === "string") {
+        const mediaType = ns.getMergedTraits().mediaType;
+
+        if (mediaType) {
+          const isJson = mediaType === "application/json" || mediaType.endsWith("+json");
+          if (isJson) {
+            return quote(LazyJsonString.from(value).toString());
+          }
+        }
+      }
+    }
+
+    if (value instanceof NumericValue) {
+      // ns can be BigDecimal or Document.
+      return value.string;
+    }
+
+    if (isObject) {
+      return this.writeObject(ns, value);
+    }
+
+    return typeof value === "string" ? quote(value) : String(value);
+  }
+}
diff --git a/packages/core/src/submodules/protocols/test-schema.spec.ts b/packages/core/src/submodules/protocols/test-schema.spec.ts
index 81d7e557910c..2bd3ebcda592 100644
--- a/packages/core/src/submodules/protocols/test-schema.spec.ts
+++ b/packages/core/src/submodules/protocols/test-schema.spec.ts
@@ -6,6 +6,8 @@ import type {
   StaticListSchema,
   StaticOperationSchema,
   StaticStructureSchema,
+  StringSchema,
+  TimestampDefaultSchema,
   TimestampEpochSecondsSchema,
 } from "@smithy/types";
 import { describe, test as it } from "vitest";
@@ -34,6 +36,43 @@ export const widget = [
   ],
 ] satisfies StaticStructureSchema;
 
+export const nestingWidget: StaticStructureSchema = [
+  3,
+  "ns",
+  "Struct",
+  0,
+  ["string", "date", "blob", "number", "list", "map", "nested"],
+  [
+    0 satisfies StringSchema,
+    4 satisfies TimestampDefaultSchema,
+    21 satisfies BlobSchema,
+    1 satisfies NumericSchema,
+    64 | 1,
+    128 | 0,
+    () => nestingWidget,
+  ],
+];
+
+export function createNestingWidget(nesting = 0) {
+  const object = {
+    string: "hello, world",
+    number: 100000,
+    list: [1, 2, 3, 4, 5, 6, 7, 8, 9],
+    map: {
+      a: "A",
+      b: "B",
+      c: "C",
+    },
+    date: new Date(0),
+    blob: new Uint8Array([0, 1, 2, 3, 4, 5, 6, 7]),
+    nested: undefined,
+  } as any;
+  if (nesting > 0) {
+    object.nested = createNestingWidget(nesting - 1);
+  }
+  return object;
+}
+
 export const deleteObjects: StaticOperationSchema = [
   9,
   "ns",
diff --git a/packages/core/src/submodules/protocols/xml/XmlShapeDeserializer.spec.ts b/packages/core/src/submodules/protocols/xml/XmlShapeDeserializer.spec.ts
index 7bdfd27f0ec4..41e92bc43427 100644
--- a/packages/core/src/submodules/protocols/xml/XmlShapeDeserializer.spec.ts
+++ b/packages/core/src/submodules/protocols/xml/XmlShapeDeserializer.spec.ts
@@ -2,10 +2,28 @@ import { NumericValue } from "@smithy/core/serde";
 import type { TimestampDateTimeSchema } from "@smithy/types";
 import { describe, expect, test as it } from "vitest";
 
-import { widget } from "../test-schema.spec";
+import { createNestingWidget, nestingWidget, widget } from "../test-schema.spec";
 import { XmlShapeDeserializer } from "./XmlShapeDeserializer";
+import { XmlShapeSerializer } from "./XmlShapeSerializer";
+
+describe(XmlShapeDeserializer.name, () => {
+  const deserializer = new XmlShapeDeserializer({
+    httpBindings: true,
+    serviceNamespace: "namespace",
+    timestampFormat: { default: 5 satisfies TimestampDateTimeSchema, useTrait: true },
+    xmlNamespace: "namespace",
+  });
+  const serializer = new XmlShapeSerializer({
+    xmlNamespace: "namespace",
+    serviceNamespace: "namespace",
+    timestampFormat: { default: 5 satisfies TimestampDateTimeSchema, useTrait: true },
+  });
+  serializer.setSerdeContext({
+    base64Encoder: (input: Uint8Array) => {
+      return Buffer.from(input).toString("base64");
+    },
+  } as any);
 
-describe("", () => {
   it("placeholder", async () => {
     const xml = `<Struct xmlns="namespace">
   <blob>QUFBQQ==</blob>
@@ -13,13 +31,6 @@ describe("", () => {
   <bigint>10000000000000000000000054321</bigint>
   <bigdecimal>0.10000000000000000000000054321</bigdecimal>
 </Struct>`;
-    const deserializer = new XmlShapeDeserializer({
-      httpBindings: true,
-      serviceNamespace: "namespace",
-      timestampFormat: { default: 5 satisfies TimestampDateTimeSchema, useTrait: true },
-      xmlNamespace: "namespace",
-    });
-
     const result = await deserializer.read(widget, xml);
     expect(result).toEqual({
       blob: new Uint8Array([65, 65, 65, 65]),
@@ -28,4 +39,35 @@ describe("", () => {
       bigdecimal: new NumericValue("0.10000000000000000000000054321", "bigDecimal"),
     });
   });
+
+  describe("performance baseline indicator", () => {
+    it("should deserialize XML strings", async () => {
+      const timings: string[] = [];
+      const strings = [];
+
+      // warmup
+      for (let i = 0; i < 12; ++i) {
+        const o = createNestingWidget(2 ** i);
+        serializer.write(nestingWidget, o);
+        const json = serializer.flush();
+        strings.push(json);
+        await deserializer.read(nestingWidget, json);
+      }
+
+      for (const s of strings) {
+        const A = performance.now();
+        await deserializer.read(nestingWidget, s);
+        const B = performance.now();
+
+        timings.push(`${B - A} (XML length = ${s.length}, ${s.length / 1024 / (B - A)} kb/ms)`);
+      }
+
+      /**
+       * No assertion here.
+       * In the initial dual-pass implementation,
+       * par time is 0 to 187ms for up to 905676 chars of XML. Up to 10 kb/ms. (kuhe's computer)
+       */
+      console.log("XmlShapeDeserializer performance timings", timings);
+    });
+  }, 30_000);
 });
diff --git a/packages/core/src/submodules/protocols/xml/XmlShapeSerializer.spec.ts b/packages/core/src/submodules/protocols/xml/XmlShapeSerializer.spec.ts
index 90e3280d7e4b..378ecbd0003a 100644
--- a/packages/core/src/submodules/protocols/xml/XmlShapeSerializer.spec.ts
+++ b/packages/core/src/submodules/protocols/xml/XmlShapeSerializer.spec.ts
@@ -2,23 +2,23 @@ import { NumericValue } from "@smithy/core/serde";
 import type { TimestampDateTimeSchema } from "@smithy/types";
 import { describe, expect, test as it } from "vitest";
 
-import { widget } from "../test-schema.spec";
+import { createNestingWidget, nestingWidget, widget } from "../test-schema.spec";
 import { simpleFormatXml } from "./simpleFormatXml";
 import { XmlShapeSerializer } from "./XmlShapeSerializer";
 
 describe(XmlShapeSerializer.name, () => {
-  it("serializes data to Query", async () => {
-    const serializer = new XmlShapeSerializer({
-      xmlNamespace: "namespace",
-      serviceNamespace: "namespace",
-      timestampFormat: { default: 5 satisfies TimestampDateTimeSchema, useTrait: true },
-    });
-    serializer.setSerdeContext({
-      base64Encoder: (input: Uint8Array) => {
-        return Buffer.from(input).toString("base64");
-      },
-    } as any);
+  const serializer = new XmlShapeSerializer({
+    xmlNamespace: "namespace",
+    serviceNamespace: "namespace",
+    timestampFormat: { default: 5 satisfies TimestampDateTimeSchema, useTrait: true },
+  });
+  serializer.setSerdeContext({
+    base64Encoder: (input: Uint8Array) => {
+      return Buffer.from(input).toString("base64");
+    },
+  } as any);
 
+  it("serializes data to Query", async () => {
     const data = {
       timestamp: new Date(0),
       bigint: 10000000000000000000000054321n,
@@ -42,4 +42,37 @@ describe(XmlShapeSerializer.name, () => {
   </bigdecimal>
 </Struct>`);
   });
+
+  describe("performance baseline indicator", () => {
+    it("should serialize objects", () => {
+      const timings: string[] = [];
+      const objects = [];
+
+      // warmup
+      for (let i = 0; i < 13; ++i) {
+        const o = createNestingWidget(2 ** i);
+        objects.push(o);
+        serializer.write(nestingWidget, o);
+        serializer.flush();
+      }
+
+      for (let i = 0; i < objects.length; ++i) {
+        const o = objects[i];
+
+        const A = performance.now();
+        serializer.write(nestingWidget, o);
+        const serialization = serializer.flush();
+        const B = performance.now();
+
+        timings.push(`${B - A} (XML length = ${serialization.length}, ${serialization.length / 1024 / (B - A)} kb/ms)`);
+      }
+
+      /**
+       * No assertion here.
+       * In the initial dual-pass implementation,
+       * par time is 0 to 600ms for up to 1810892 chars of XML. Up to 28 kb/ms. (kuhe's computer)
+       */
+      console.log("XmlShapeSerializer performance timings", timings);
+    });
+  }, 30_000);
 });