The slowness appears to be caused by continual directory scanning. For me at least, turning off the "Refresh when files change" option, and so having to do "view, refresh" or Command-R, made SourceTree be zippy again.
And as I'm usually on a laptop, having less continued CPU usage is a good tradeoff against having to press refresh when I'm going to do something in SourceTree.
And as I'm usually on a laptop, having less continued CPU usage is a good tradeoff against having to press refresh when I'm going to do something in SourceTree.