SourceTree went downhill after the last major UI overhaul and never really recovered. I used to love it for more involved commits (picking apart lines and hunks, etc), but now it's slow and clunky, even on nice hardware.
But it's become unbearably slow. I really wish they'd fix this.
Unfortunately, I don't really find a good GUI alternative that I like either ...
It's fast, it's regularly updated and supported, and it's powerful enough to do complex stuff (rebases, merges, cherry-picks, etc).
With TortoiseGit, when I find a git repo in the file-system and want to start working with it, I don't have to start another program, I can just activate the file browser's context menu with a single keystroke and then I can instantly see all of the Git commands that I normally use. I hardly even have to look at the list of commands though because the next keystroke is usually to hit the letter of the command that I want: (M)erge, (C)ommit, S(w)itch/Checkout, (L)og, etc... because you can customize the context menu to show the most useful commands.
(Disabling overlay icons in TortoiseGit is also a good idea. I typically just delete their registry entries via SysInternals/Autoruns.)
I have recently tried Tower and it provides similar functionality without the performance issues.