Hardware and performance

In my 2.4 site the javascript "head" file (and the "footer" one, though it doesn't block renderering as much since it's at the bottom of the page) are actually empty. I filed a bug, number MDL-35230 which claims to be fixed, so you shouldn't have an empty file anymore. If you follow that link and actually see javascript, then you might want to consider moving it to the footer javascript (and if it's empty then perhaps that bug isn't as fixed as it claims).

Some of the things you can do to make the theme CSS smaller, and therefore faster is to for example, manually delete RTL rules if you're not using them, delete Moodle modules you're not using (since Moodle will stick the CSS in anyway), run the CSS through a reformatter to make it more compressible. There are various bugs filed about these issues too, mostly linked from MDL-39094

If you're handy then some of them can be done manually to trim down the CSS size.