Bugfix/Performance: Start the modern child compiler earlier (#9257)

* Start the modern Child Compiler earlier during builds

* fix child compiler errors not bubbling

packages/next/build/webpack/plugins/next-esm-plugin.ts

@@ -254,24 +254,36 @@ export default class NextEsmPlugin implements Plugin {
       }
     }
 
-    compilation.hooks.additionalAssets.tapAsync(
-      PLUGIN_NAME,
-      childProcessDone => {
-        this.updateOptions(childCompiler)
-
-        childCompiler.runAsChild((err, entries, childCompilation) => {
-          if (err) {
-            return childProcessDone(err)
-          }
-
-          if (childCompilation.errors.length > 0) {
-            return childProcessDone(childCompilation.errors[0])
-          }
-
-          this.updateAssets(compilation, childCompilation)
-          childProcessDone()
-        })
-      }
-    )
+    // Hold back the main compilation until our Child Compiler has completed so its assets get optimized
+    const child = new Promise((resolve, reject) => {
+      // Defer the child compiler until known main thread "dead time" (while Terser is doing minification in the background)
+      let started = false
+      compilation.hooks.optimizeChunkAssets.intercept({
+        call: () => {
+          // only run the first time optimizeChunkAssets is called
+          if (started) return
+          started = true
+
+          // Delay the Child Compiler until optimizeChunkAssets has had time to send work to the Terser pool
+          setTimeout(() => {
+            this.updateOptions(childCompiler)
+
+            childCompiler.runAsChild((err, entries, childCompilation) => {
+              if (err) {
+                return reject(err)
+              }
+
+              if (childCompilation.errors.length > 0) {
+                return reject(childCompilation.errors[0])
+              }
+
+              this.updateAssets(compilation, childCompilation)
+              resolve()
+            })
+          }, 500)
+        },
+      })
+    })
+    compilation.hooks.optimizeAssets.tapPromise(PLUGIN_NAME, () => child)
   }
 }