Performance: Tab group function is called too often

[chris-martin]

My config groups tabs by git repo, by searching up the filesystem hierarchy looking for a .git directory (this is one of the suggestions on the emacs wiki. Today I opened a file from an SSH mount, and noticed a huge performance degradation.

The source of the slowdown was the tab group function, which is apparently invoked for every buffer every time the cursor moves. This seems excessive.

I've memoized my tabbar-buffer-groups-function as a workaround.