When using the mw.loader.using() method, The modules are requested in synchronous mode, because inside the using function, the request() call is done without specifying the mode, so the modules will be requested one by one, the script waiting for one module to be ready before calling the next one.
Perhaps this can be usefull for some purpose, but for some other purpose, when you have to call several script subpages before launching something and if the loading order doesn't matter, the result is a waste of time and a very slow loading.
I ask for a fix in mw.loader.using method, with a new parameter (async ?) to be passed to the request() call.
See : fr:wp:Special:Diff/113497286.