CPanel and WHM are the heaviest sites on my server.
I think some resources could be saved by those simple steps
- separating all style in external CSS (static files)
- separating all JS logic in other static file(s) (no inline JS)
- maybe using JQuery as JS framework (once in a while I have some Ajax errors in CPanel)

I know this sounds picky but any free resource is worth when talking about a super popular control panel