I use tig (ncurses) but I think this is also a bit slow in Haiku (I use it with WebKit which is a very large repo, however).
I think part of the slowness is because git itself normally relies on hardlinks and uses a slower fallback when used on bfs disks (which can’t do hardlinks). But I have not confirmed if using git with another filesystem in Haiku helps with that.
I actually tried porting SDL-Tk so that the built-in gitk and git gui would work, as these work well on other OSes, but the port just crashes in an assert, or (if you continue past that) displays a white screen. I committed the recipe, though, so if you want to look into that, you may find it useful.