diff --git a/packages/metro-source-map/src/composeSourceMaps.js b/packages/metro-source-map/src/composeSourceMaps.js
index c512b8d844..a0e8967c5c 100644
--- a/packages/metro-source-map/src/composeSourceMaps.js
+++ b/packages/metro-source-map/src/composeSourceMaps.js
@@ -10,11 +10,17 @@
  */
 
 import type {SourcePosition} from './Consumer/types';
-import type {IConsumer, MixedSourceMap} from './source-map';
+import type {
+  FBSourcesArray,
+  HermesFunctionOffsets,
+  IConsumer,
+  MixedSourceMap,
+} from './source-map';
 import type {Number0, Number1} from 'ob1';
 
+import B64Builder from './B64Builder';
 import Consumer from './Consumer';
-import {SourceMapGenerator} from 'source-map';
+import {get0, get1} from 'ob1';
 
 // TODO(t67648443): Bypass the `sort-requires` rule for this file because of a dependency cycle.
 Consumer;
@@ -41,45 +47,189 @@ export default function composeSourceMaps(
     })
     .reverse();
 
-  const generator = new SourceMapGenerator({
-    file: consumers[0].file,
-  });
+  const sources: Array<string> = [];
+  const names: Array<string> = [];
+  const sourceIndices: Map<string, number> = new Map();
+  const nameIndices: Map<string, number> = new Map();
+
+  // The composed mappings as parallel arrays, in the order the last map
+  // yields them. An unmapped position has source index -1 and original
+  // line and column 0.
+  const generatedLines: Array<number> = [];
+  const generatedColumns: Array<number> = [];
+  const mappingSources: Array<number> = [];
+  const originalLines: Array<number> = [];
+  const originalColumns: Array<number> = [];
+  const mappingNames: Array<number> = [];
+  let sorted = true;
 
   const earlierConsumers = consumers.slice(1);
   consumers[0].eachMapping(mapping => {
-    const original =
-      mapping.originalLine == null || mapping.originalColumn == null
-        ? {line: null, column: null, source: null, name: null}
-        : earlierConsumers.length === 0
-          ? {
-              line: mapping.originalLine,
-              column: mapping.originalColumn,
-              source: mapping.source,
-              name: mapping.name,
-            }
-          : findOriginalPosition(
-              earlierConsumers,
-              mapping.originalLine,
-              mapping.originalColumn,
-            );
-    generator.addMapping({
-      generated: {
-        line: mapping.generatedLine,
-        column: mapping.generatedColumn,
-      },
-      original:
-        original.line != null
-          ? {
-              line: original.line,
-              column: original.column,
-            }
-          : null,
-      source: original.source,
-      name: original.name,
-    });
+    let originalLine: ?number = null;
+    let originalColumn: ?number = null;
+    let source: ?string = null;
+    let name: ?string = null;
+    const mappingOriginalLine = mapping.originalLine;
+    const mappingOriginalColumn = mapping.originalColumn;
+    if (mappingOriginalLine != null && mappingOriginalColumn != null) {
+      if (earlierConsumers.length === 0) {
+        originalLine = get1(mappingOriginalLine);
+        originalColumn = get0(mappingOriginalColumn);
+        source = mapping.source;
+        name = mapping.name;
+      } else {
+        const original = findOriginalPosition(
+          earlierConsumers,
+          mappingOriginalLine,
+          mappingOriginalColumn,
+        );
+        originalLine = original.line;
+        originalColumn = original.column;
+        source = original.source;
+        name = original.name;
+      }
+    }
+    const generatedLine = get1(mapping.generatedLine);
+    const generatedColumn = get0(mapping.generatedColumn);
+    const hasOriginal = originalLine != null;
+    if (!(
+      generatedLine > 0 &&
+      generatedColumn >= 0 &&
+      (originalLine != null
+        ? originalLine > 0 && (originalColumn ?? 0) >= 0 && source
+        : !source && !name)
+    )) {
+      throw new Error(
+        'Invalid mapping: ' +
+          JSON.stringify({
+            generated: {line: generatedLine, column: generatedColumn},
+            source,
+            original: hasOriginal
+              ? {line: originalLine, column: originalColumn}
+              : null,
+            name,
+          }),
+      );
+    }
+
+    const count = generatedLines.length;
+    if (
+      count > 0 &&
+      (generatedLine < generatedLines[count - 1] ||
+        (generatedLine === generatedLines[count - 1] &&
+          generatedColumn < generatedColumns[count - 1]))
+    ) {
+      sorted = false;
+    }
+    generatedLines.push(generatedLine);
+    generatedColumns.push(generatedColumn);
+    if (originalLine != null && source != null) {
+      let sourceIndex = sourceIndices.get(source);
+      if (sourceIndex == null) {
+        sourceIndex = sources.length;
+        sources.push(source);
+        sourceIndices.set(source, sourceIndex);
+      }
+      mappingSources.push(sourceIndex);
+      originalLines.push(originalLine);
+      originalColumns.push(originalColumn ?? 0);
+    } else {
+      mappingSources.push(-1);
+      originalLines.push(0);
+      originalColumns.push(0);
+    }
+    if (name != null) {
+      let nameIndex = nameIndices.get(name);
+      if (nameIndex == null) {
+        nameIndex = names.length;
+        names.push(name);
+        nameIndices.set(name, nameIndex);
+      }
+      mappingNames.push(nameIndex);
+    } else {
+      mappingNames.push(-1);
+    }
   });
 
-  const composedMap = generator.toJSON();
+  // Orders two composed mappings by generated position, then source,
+  // original position and name.
+  const compare = (a: number, b: number): number =>
+    generatedLines[a] - generatedLines[b] ||
+    generatedColumns[a] - generatedColumns[b] ||
+    compareStrings(
+      mappingSources[a] === -1 ? null : sources[mappingSources[a]],
+      mappingSources[b] === -1 ? null : sources[mappingSources[b]],
+    ) ||
+    originalLines[a] - originalLines[b] ||
+    originalColumns[a] - originalColumns[b] ||
+    compareStrings(
+      mappingNames[a] === -1 ? null : names[mappingNames[a]],
+      mappingNames[b] === -1 ? null : names[mappingNames[b]],
+    );
+
+  const count = generatedLines.length;
+  let order: ?Array<number> = null;
+  if (!sorted) {
+    order = Array.from({length: count}, (_, i) => i);
+    order.sort(compare);
+  }
+
+  // Encode the mappings, dropping any identical to the one before it.
+  const builder = new B64Builder();
+  let previousIndex = -1;
+  let previousGeneratedLine = 1;
+  let previousGeneratedColumn = 0;
+  let previousSource = 0;
+  let previousOriginalLine = 0;
+  let previousOriginalColumn = 0;
+  let previousName = 0;
+  for (let position = 0; position < count; position++) {
+    const index = order == null ? position : order[position];
+    const generatedLine = generatedLines[index];
+    if (generatedLine !== previousGeneratedLine) {
+      builder.markLines(generatedLine - previousGeneratedLine);
+      previousGeneratedLine = generatedLine;
+      previousGeneratedColumn = 0;
+    } else if (previousIndex !== -1 && compare(index, previousIndex) === 0) {
+      previousIndex = index;
+      continue;
+    }
+    previousIndex = index;
+
+    builder.startSegment(generatedColumns[index] - previousGeneratedColumn);
+    previousGeneratedColumn = generatedColumns[index];
+    const sourceIndex = mappingSources[index];
+    if (sourceIndex !== -1) {
+      builder.append(sourceIndex - previousSource);
+      previousSource = sourceIndex;
+      // Original lines are 0-based in the encoding.
+      builder.append(originalLines[index] - 1 - previousOriginalLine);
+      previousOriginalLine = originalLines[index] - 1;
+      builder.append(originalColumns[index] - previousOriginalColumn);
+      previousOriginalColumn = originalColumns[index];
+      const nameIndex = mappingNames[index];
+      if (nameIndex !== -1) {
+        builder.append(nameIndex - previousName);
+        previousName = nameIndex;
+      }
+    }
+  }
+
+  const composedMap: {
+    version: number,
+    sources: Array<string>,
+    names: Array<string>,
+    mappings: string,
+    file?: string,
+    sourcesContent?: Array<?string>,
+    x_facebook_sources?: FBSourcesArray,
+    x_hermes_function_offsets?: HermesFunctionOffsets,
+    x_google_ignoreList?: Array<number>,
+  } = {version: 3, sources, names, mappings: builder.toString()};
+  const {file} = consumers[0];
+  if (file != null) {
+    composedMap.file = file;
+  }
 
   composedMap.sourcesContent = composedMap.sources.map(source =>
     consumers[consumers.length - 1].sourceContentFor(source, true),
@@ -103,6 +253,14 @@ export default function composeSourceMaps(
   return composedMap;
 }
 
+function compareStrings(a: ?string, b: ?string): number {
+  if (a === b) {
+    return 0;
+  }
+  // $FlowFixMe[invalid-compare] Matches `source-map`'s ordering, null included
+  return a > b ? 1 : -1;
+}
+
 function findOriginalPosition(
   consumers: ReadonlyArray<IConsumer>,
   generatedLine: Number1,