diff --git a/apps/api-extractor/src/analyzer/AstSymbolTable.ts b/apps/api-extractor/src/analyzer/AstSymbolTable.ts
index 1bfa982b57..b5f47c751a 100644
--- a/apps/api-extractor/src/analyzer/AstSymbolTable.ts
+++ b/apps/api-extractor/src/analyzer/AstSymbolTable.ts
@@ -129,6 +129,25 @@ export class AstSymbolTable {
     return this._exportAnalyzer.fetchAstModuleExportInfo(astModule);
   }
 
+  /**
+   * Returns the set of source files that contain a declaration that was analyzed, i.e. that is
+   * reachable from the entry point.
+   *
+   * @remarks
+   * API Extractor analyzes the compiler's `.d.ts` outputs, and by default does not use
+   * `skipLibCheck`, so type-checking the entire program would bind-and-check every transitively
+   * reachable declaration file -- including deep dependencies that are not part of the API surface.
+   * This set allows compiler diagnostics to be scoped to just the files that contribute
+   * declarations to the analyzed API.
+   */
+  public collectAnalyzedSourceFiles(): Set<ts.SourceFile> {
+    const sourceFiles: Set<ts.SourceFile> = new Set<ts.SourceFile>();
+    for (const declaration of this._astDeclarationsByDeclaration.keys()) {
+      sourceFiles.add(declaration.getSourceFile());
+    }
+    return sourceFiles;
+  }
+
   /**
    * Attempts to retrieve an export by name from the specified `AstModule`.
    * Returns undefined if no match was found.
diff --git a/apps/api-extractor/src/analyzer/TypeScriptInternals.ts b/apps/api-extractor/src/analyzer/TypeScriptInternals.ts
index 88059f26e8..07fe3d5547 100644
--- a/apps/api-extractor/src/analyzer/TypeScriptInternals.ts
+++ b/apps/api-extractor/src/analyzer/TypeScriptInternals.ts
@@ -139,7 +139,24 @@ export class TypeScriptInternals {
     if (!typeChecker.getEmitResolver) {
       throw new InternalError('Missing TypeChecker.getEmitResolver');
     }
-    const resolver: any = typeChecker.getEmitResolver();
+
+    // We only use `EmitResolver.hasGlobalName`, which simply queries the type checker's `globals`
+    // symbol table.  That table (including ambient declarations, `jsGlobalAugmentations`, and
+    // `declare global` augmentations) is populated synchronously when the checker is created, so it
+    // does not require type checking to have run.
+    //
+    // By default, `getEmitResolver()` forces a full-program diagnostic type-check before returning
+    // the resolver.  Since API Extractor otherwise only type-checks the source files that are
+    // reachable from the entry point (see `Collector.analyze`), that full check would dominate the
+    // analysis cost.  Passing `skipDiagnostics: true` avoids it while still returning a resolver
+    // whose `hasGlobalName` behaves identically.  The parameter is a compiler internal; older
+    // TypeScript versions that lack it simply ignore the extra argument (falling back to the
+    // previous, slower behavior).
+    const resolver: any = typeChecker.getEmitResolver(
+      /*sourceFile*/ undefined,
+      /*cancellationToken*/ undefined,
+      /*skipDiagnostics*/ true
+    );
     if (!resolver.hasGlobalName) {
       throw new InternalError('Missing EmitResolver.hasGlobalName');
     }
diff --git a/apps/api-extractor/src/collector/Collector.ts b/apps/api-extractor/src/collector/Collector.ts
index 2e6a374f11..44b0ef3cc9 100644
--- a/apps/api-extractor/src/collector/Collector.ts
+++ b/apps/api-extractor/src/collector/Collector.ts
@@ -200,13 +200,6 @@ export class Collector {
       throw new Error('DtsRollupGenerator.analyze() was already called');
     }
 
-    // This runs a full type analysis, and then augments the Abstract Syntax Tree (i.e. declarations)
-    // with semantic information (i.e. symbols).  The "diagnostics" are a subset of the everyday
-    // compile errors that would result from a full compilation.
-    for (const diagnostic of this._program.getSemanticDiagnostics()) {
-      this.messageRouter.addCompilerDiagnostic(diagnostic);
-    }
-
     const sourceFiles: readonly ts.SourceFile[] = this.program.getSourceFiles();
 
     if (this.messageRouter.showDiagnostics) {
@@ -294,6 +287,25 @@ export class Collector {
       }
     }
 
+    // Report compiler diagnostics, but only for the source files that are reachable from the entry
+    // point: the files that contribute an analyzed declaration, plus the intermediate re-export
+    // files that were visited while resolving exports.  API Extractor analyzes the compiler's `.d.ts`
+    // outputs and (by default) does not enable `skipLibCheck`, so running `getSemanticDiagnostics()`
+    // across the entire program would bind-and-check every transitively reachable declaration file,
+    // including deep dependencies that are not part of the API surface.  Scoping the diagnostics to
+    // the analyzed files avoids that cost while still surfacing every error that pertains to the
+    // exported API.  (Binding, which the analysis relies on, happens eagerly for all files when the
+    // type checker is created, so this does not affect the analysis itself.)
+    const diagnosticSourceFiles: Set<ts.SourceFile> = this.astSymbolTable.collectAnalyzedSourceFiles();
+    for (const sourceFile of nonExternalSourceFiles) {
+      diagnosticSourceFiles.add(sourceFile);
+    }
+    for (const sourceFile of diagnosticSourceFiles) {
+      for (const diagnostic of this._program.getSemanticDiagnostics(sourceFile)) {
+        this.messageRouter.addCompilerDiagnostic(diagnostic);
+      }
+    }
+
     // Here, we're collecting reference directives from all non-external source files
     // that were encountered while looking for exports, but only those references that
     // were explicitly written by the developer and marked with the `preserve="true"`
diff --git a/common/changes/@microsoft/api-extractor/optimize-typecheck_2026-07-21-22-06-48.json b/common/changes/@microsoft/api-extractor/optimize-typecheck_2026-07-21-22-06-48.json
new file mode 100644
index 0000000000..57a0061195
--- /dev/null
+++ b/common/changes/@microsoft/api-extractor/optimize-typecheck_2026-07-21-22-06-48.json
@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "comment": "Improve analysis performance by scoping compiler diagnostics to the source files that are reachable from the entry point, rather than type-checking the entire program. As a result, compiler errors are now only reported for declarations that contribute to the analyzed API surface.",
+      "type": "minor",
+      "packageName": "@microsoft/api-extractor"
+    }
+  ],
+  "packageName": "@microsoft/api-extractor",
+  "email": "dmichon-msft@users.noreply.github.com"
+}