Flarum is fast because the js and css are compiled and minified. This reduces overall overhead considerably. In addition the assets for the forum and admin are split up to improve loading times for non-admin users. As it's also a SPA these resources have to be loaded only once until the next page refresh.
Splitting up the js, css - for instance - based on index, show etc would increase the number of necessary files to download from one hostname and will consequentially reduce the number of available parallel connections available with http/1.1 to download for instance avatars, uploaded images etc.
I'm not sure your suggestion will work for us.
