[BUGFIX PERF] don’t rebuild a module that has already been built, bec…

…ause that is very slow.

lib/loader/loader.js

@@ -127,7 +127,7 @@ var loader, define, requireModule, require, requirejs;
   };
 
   Module.prototype.build = function() {
-    if (this.state === FAILED) { return; }
+    if (this.state === FAILED || this.state === LOADED) { return; }
     this.state = FAILED;
     this.reify()
     this.exports();