Every version after 3.4.20 (which works) suffer this same issue:

Every time sourctree needs to load some commits, in other words this appears:
"Loading 100 commits to reach commit X" , sourctree gets permanently stuck doing commit load. What is worst the window becomes "not responding" and stuck. But occasionally you have few seconds time to press cancel button on the load bar and sourcetree recovers then.

This repeats on 2 different computers with exactly same behavior.

Result: All versions after 3.4.20 have been unusable.