This one had annoyed me for quite some time, very happy to finally have found a fix:

Every time Gmail refreshed, it would reload all the star icons one by one, and would freeze up that tab until all the visible star icons were reloaded. This would make me wait for all the stars to load before I could go back to work on another tab...BOOO.

Here's the fix:

1) In the location bar, go to about:config
2) Search for browser.cache.memory.enable
3) Change it to "true"