The source for Unreal Engine is extremely large (80 GB). If I open the bookmark, Sourcetree spawns a git process, and I see the spinny icon of doom running. Eventually, Sourcetree may populate a handful of elements, but the spinny icon of doom continues.

I eventually have to manually kill the git process after it's been running for multiple days. If I close the bookmark and reopen, I now have two git processes spinning into oblivion.