diff --git a/code/builders/builder-webpack5/src/plugins/webpack-inject-mocker-runtime-plugin.ts b/code/builders/builder-webpack5/src/plugins/webpack-inject-mocker-runtime-plugin.ts
index da843d1cd782..84e0fcdfdac6 100644
--- a/code/builders/builder-webpack5/src/plugins/webpack-inject-mocker-runtime-plugin.ts
+++ b/code/builders/builder-webpack5/src/plugins/webpack-inject-mocker-runtime-plugin.ts
@@ -13,6 +13,7 @@ const PLUGIN_NAME = 'WebpackInjectMockerRuntimePlugin';
  * Storybook preview bundle, are executed.
  */
 export class WebpackInjectMockerRuntimePlugin {
+  private cachedRuntime: string | null = null;
   // We need to lazy-require HtmlWebpackPlugin because it's an optional peer dependency.
   private getHtmlWebpackPlugin(compiler: Compiler): typeof HtmlWebpackPlugin | null {
     try {
@@ -52,20 +53,23 @@ export class WebpackInjectMockerRuntimePlugin {
         PLUGIN_NAME,
         (data, cb) => {
           try {
-            const runtimeScriptContent = getMockerRuntime();
+            const runtimeScriptContent =
+              this.cachedRuntime ?? (this.cachedRuntime = getMockerRuntime());
             const runtimeAssetName = 'mocker-runtime-injected.js';
 
             // Use the documented `emitAsset` method to add the pre-bundled runtime script
             // to the compilation's assets. This is the standard Webpack way.
-            compilation.emitAsset(
-              runtimeAssetName,
-              new compiler.webpack.sources.RawSource(runtimeScriptContent)
-            );
+            if (!compilation.getAsset(runtimeAssetName)) {
+              compilation.emitAsset(
+                runtimeAssetName,
+                new compiler.webpack.sources.RawSource(runtimeScriptContent)
+              );
+              data.assets.js.unshift(runtimeAssetName);
+            }
 
-            // Prepend the name of our new asset to the list of JavaScript files.
+            // Prepend the name of our new asset to the list of JavaScript files, once.
             // HtmlWebpackPlugin will automatically create a <script> tag for it
             // and place it at the beginning of the body scripts.
-            data.assets.js.unshift(runtimeAssetName);
             cb(null, data);
           } catch (error) {
             // In case of an error (e.g., file not found), pass it to Webpack's compilation.
diff --git a/code/builders/builder-webpack5/src/plugins/webpack-mock-plugin.ts b/code/builders/builder-webpack5/src/plugins/webpack-mock-plugin.ts
index dd53310fd231..9ab673d3d21d 100644
--- a/code/builders/builder-webpack5/src/plugins/webpack-mock-plugin.ts
+++ b/code/builders/builder-webpack5/src/plugins/webpack-mock-plugin.ts
@@ -9,6 +9,7 @@ import {
   resolveExternalModule,
   resolveWithExtensions,
 } from 'storybook/internal/mocking-utils';
+import { logger } from 'storybook/internal/node-logger';
 
 import type { Compiler } from 'webpack';
 
@@ -49,6 +50,8 @@ const PLUGIN_NAME = 'storybook-mock-plugin';
 export class WebpackMockPlugin {
   private readonly options: WebpackMockPluginOptions;
   private mockMap: Map<string, ResolvedMock> = new Map();
+  private candidateSpecifiers: Set<string> = new Set();
+  private lastPreviewMtime: number | undefined;
 
   constructor(options: WebpackMockPluginOptions) {
     if (!options.previewConfigPath) {
@@ -63,20 +66,29 @@ export class WebpackMockPlugin {
    * @param {Compiler} compiler The Webpack compiler instance.
    */
   public apply(compiler: Compiler): void {
-    const logger = compiler.getInfrastructureLogger(PLUGIN_NAME);
-
     // This function will be called to update the mock map before each compilation.
     const updateMocks = () => {
+      const mTimePreviewConfig = this.getPreviewConfigMtime(compiler);
+      if (
+        this.lastPreviewMtime &&
+        mTimePreviewConfig &&
+        mTimePreviewConfig <= this.lastPreviewMtime
+      ) {
+        return; // unchanged
+      }
+      const resolved = this.extractAndResolveMocks(compiler);
       this.mockMap = new Map(
-        this.extractAndResolveMocks(compiler).flatMap((mock) => [
-          // first one, full path
+        resolved.flatMap((mock) => [
           [mock.absolutePath, mock],
-          // second one, without the extension
           [mock.absolutePath.replace(/\.[^.]+$/, ''), mock],
         ])
       );
-      // divide by 2 because we add both the full path and the path without the extension
-      logger.info(`Mock map updated with ${this.mockMap.size / 2} mocks.`);
+      this.candidateSpecifiers = new Set(resolved.map((m) => m.path));
+      this.lastPreviewMtime = mTimePreviewConfig;
+
+      if (resolved.length > 0) {
+        logger.info(`Mock map updated with ${resolved.length} mocks.`);
+      }
     };
 
     compiler.hooks.beforeRun.tap(PLUGIN_NAME, updateMocks); // for build
@@ -85,10 +97,17 @@ export class WebpackMockPlugin {
     // Apply the replacement plugin. Its callback will now use the dynamically updated mockMap.
     new compiler.webpack.NormalModuleReplacementPlugin(/.*/, (resource) => {
       try {
+        if (this.mockMap.size === 0) {
+          return;
+        }
         const path = resource.request;
         const importer = resource.context;
 
         const isExternal = getIsExternal(path, importer);
+        // Early filter only for external specifiers. Relative/local specifiers need resolution
+        if (isExternal && !this.candidateSpecifiers.has(path)) {
+          return;
+        }
         const absolutePath = isExternal
           ? resolveExternalModule(path, importer)
           : resolveWithExtensions(path, importer);
@@ -115,6 +134,16 @@ export class WebpackMockPlugin {
     });
   }
 
+  private getPreviewConfigMtime(compiler: Compiler): number | undefined {
+    try {
+      const fs = compiler.inputFileSystem as any;
+      const stat = fs.statSync?.(this.options.previewConfigPath);
+      return stat?.mtime?.getTime?.();
+    } catch {
+      return undefined;
+    }
+  }
+
   /**
    * Reads the preview config, parses it to find all `sb.mock()` calls, and resolves their
    * corresponding mock implementations.