I use Sublime Merge daily at my job, but over weeks (or maybe months) it tends to get really slow on certain operations such as doing a rebase.
I think this might be due to it never triggering any cleanup on the git repo, because today it took 30s per commit while rebasing and after I went to redo it on the command line, just to check if it was the same it told me (after I reset my branch):
Auto packing the repository in background for optimum performance.
See "git help gc" for manual housekeeping.
And after that trying the rebase again in Sublime Merge it was 10x faster.