ECMAScript 2015 promises are well supported on most browsers. Should you need to support an older browser then there are some great shims out there that can add the functionality with a minimum of overhead.

When examining the performance of retrieving JavaScript from a remote server, there are tools provided in most modern browsers for viewing a timeline of resource loading. This timeline will show when the browser is waiting for scripts to be downloaded and when it is parsing the scripts. Using this timeline allows for experimenting to find the best way to load a script or series of scripts.