Manual:$wgResourceLoaderStorageEnabled/en

Details
If set to true, ResourceLoader modules are cached in  in modern browsers and loaded with. This allows invalidating a module that has been updated without invalidating the other modules, even if they are all loaded as a single minified-concatenated file.

This will speed up page loading on wikis where the JS code changes frequently. It might cause weird behavior in browser developer toolbars (such as some source files not showing up) so you might want to disable it on local developer wikis used for debugging.