So out of curiousity I checked how much stuff is cached when I refresh a notebook:
Only 25.4 KiB, not bad! And 21.5 KiB of that is the favicon.
So I downloaded that and threw it through ImageOptimizer, now it’s 8,5 KiB. Woohoo!
(Yes, I know that this topic and that screenshot up there alone means this won’t pay back for itself in terms of savings before the heat death of the universe happens. I’m just having some fun :P)