diff --git a/packages/angular_devkit/build_angular/src/webpack/plugins/devtools-ignore-plugin.ts b/packages/angular_devkit/build_angular/src/webpack/plugins/devtools-ignore-plugin.ts
index 0840fb21566f..85bee65bac1f 100644
--- a/packages/angular_devkit/build_angular/src/webpack/plugins/devtools-ignore-plugin.ts
+++ b/packages/angular_devkit/build_angular/src/webpack/plugins/devtools-ignore-plugin.ts
@@ -33,13 +33,14 @@ export class DevToolsIgnorePlugin {
   constructor(private options: Options) {}
 
   apply(compiler: Compiler) {
-    const { SourceMapSource } = compiler.webpack.sources;
+    const { RawSource } = compiler.webpack.sources;
 
     compiler.hooks.compilation.tap(PLUGIN_NAME, (compilation) => {
       compilation.hooks.processAssets.tap(
         {
           name: PLUGIN_NAME,
           stage: Compilation.PROCESS_ASSETS_STAGE_DEV_TOOLING,
+          additionalAssets: true,
         },
         (assets) => {
           let vendorsTest: RegExp | null = null;
@@ -55,16 +56,25 @@ export class DevToolsIgnorePlugin {
           }
 
           for (const [name, asset] of Object.entries(assets)) {
-            const map = asset.map() as Object & SourceMap;
-            if (!map) {
+            // Instead of using `asset.map` we process the map files RawSource.
+            // This is because `.map()` is slow and take several seconds.
+            if (!name.endsWith('.map')) {
+              // Ignore non map files
               continue;
             }
 
+            const mapContent = asset.source().toString();
+            if (!mapContent) {
+              continue;
+            }
+
+            const map = JSON.parse(mapContent) as SourceMap;
+
             map[IGNORE_LIST] = Object.entries(map.sources)
               .filter(([, source]) => vendorsTest?.test(source) || runtimeTest?.test(source))
               .map(([index]) => +index);
 
-            compilation.updateAsset(name, new SourceMapSource(asset.source(), name, map));
+            compilation.updateAsset(name, new RawSource(JSON.stringify(map, undefined, 2)));
           }
         },
       );