perf: fix performance issue caused by refactoring

src/api/install.ts

@@ -92,17 +92,20 @@ async function installInContext (installType: string, packagesToInstall: Depende
         cacheTTL: opts.cacheTTL
       }),
     }
-    return (
-      await installMultiple(installCtx,
+    return Array.prototype.concat.apply([], await Promise.all([
+      installMultiple(
+        installCtx,
         packagesToInstall,
         nodeModulesPath,
-        installOpts)
-    ).concat(
-      await installMultiple(installCtx,
+        installOpts
+      ),
+      installMultiple(
+        installCtx,
         ctx.pkg && ctx.pkg && ctx.pkg.optionalDependencies || {},
         nodeModulesPath,
-        Object.assign({}, installOpts, {optional: true}))
-    )
+        Object.assign({}, installOpts, {optional: true})
+      ),
+    ]))
   })
 
   if (opts.flatTree) {

src/install/installMultiple.ts

@@ -92,18 +92,22 @@ async function install (pkgRawSpec: string, modules: string, ctx: InstallContext
       optional: dependency.optional,
       root: dependency.srcPath,
     })
-  dependency.dependencies = (await installMultiple(ctx,
+  dependency.dependencies = Array.prototype.concat.apply([], await Promise.all([
+    installMultiple(
+      ctx,
       dependency.pkg.dependencies || {},
       path.join(dependency.path, 'node_modules'),
-      nextInstallOpts)
-  ).concat(
-    await installMultiple(ctx,
+      nextInstallOpts
+    ),
+    installMultiple(
+      ctx,
       dependency.pkg.optionalDependencies || {},
       path.join(dependency.path, 'node_modules'),
       Object.assign({}, nextInstallOpts, {
         optional: true
-      }))
-  )
+      })
+    ),
+  ]))
   ctx.piq = ctx.piq || []
   ctx.piq.push({
     path: dependency.path,