The core is a plugin too. So imo the part of TW that can be browser cached is the boot procedure. Which is way to small, to make a difference.
As I see it, it would be a little bit more like this:
https://github.com/Jermolene/TiddlyWiki5/blob/master/boot/boot.jsEven if TW empty.html is 1.4 MByte, in reality about 300k of data will be sent, if you use a server that uses gzip compression.
So as soon as you use one image all this micro optimizations go away.
just my 2 cents
-mario