As for the slowness it is hard to say anything specific without a feeling how slow slow is ;)
It really depends on the PC also.
I am afraid but I suspect there is not much that could be contributed except the usual tips to stop other software in the background, to check if something else changed, to go back to an older version that was not so slow or to see if logs show something special.
Regards,
Daniel
I've been using Sourcetree for more than five years and it has always struggled with large repositories. In fact, it was unusable for those with more than a few thousand files. It also struggles and sometimes crashes on diffs with a few hundred files. This isn't a matter of hardware performance. It's a problem with Sourcetree.