perf: run dev.createEnvironment and build.createEnvironment concurrently (#20699)

Author: sapphi-red

packages/vite/src/node/build.ts

@@ -1612,6 +1612,7 @@ export async function createBuilder(
   if (useLegacyBuilder) {
     await setupEnvironment(config.build.ssr ? 'ssr' : 'client', config)
   } else {
+    const environmentConfigs: [string, ResolvedConfig][] = []
     for (const environmentName of Object.keys(config.environments)) {
       // We need to resolve the config again so we can properly merge options
       // and get a new set of plugins for each build environment. The ecosystem
@@ -1654,9 +1655,14 @@ export async function createBuilder(
           patchPlugins,
         )
       }
-
-      await setupEnvironment(environmentName, environmentConfig)
+      environmentConfigs.push([environmentName, environmentConfig])
     }
+    await Promise.all(
+      environmentConfigs.map(
+        async ([environmentName, environmentConfig]) =>
+          await setupEnvironment(environmentName, environmentConfig),
+      ),
+    )
   }
 
   return builder

packages/vite/src/node/server/index.ts

@@ -509,22 +509,24 @@ export async function _createServer(
 
   const environments: Record<string, DevEnvironment> = {}
 
-  for (const [name, environmentOptions] of Object.entries(
-    config.environments,
-  )) {
-    environments[name] = await environmentOptions.dev.createEnvironment(
-      name,
-      config,
-      {
-        ws,
-      },
-    )
-  }
+  await Promise.all(
+    Object.entries(config.environments).map(
+      async ([name, environmentOptions]) => {
+        const environment = await environmentOptions.dev.createEnvironment(
+          name,
+          config,
+          {
+            ws,
+          },
+        )
+        environments[name] = environment
 
-  for (const environment of Object.values(environments)) {
-    const previousInstance = options.previousEnvironments?.[environment.name]
-    await environment.init({ watcher, previousInstance })
-  }
+        const previousInstance =
+          options.previousEnvironments?.[environment.name]
+        await environment.init({ watcher, previousInstance })
+      },
+    ),
+  )
 
   // Backward compatibility