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.
Fully agree, they made a lot of nonsense changes, and from what I heard, it was the same on Mac (I'm on Windows). 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).