SourceTree appears to be slow
Problem
SourceTree appears to be slow, particularly when the refreshing views such as the history view.
Cause
The following items may be affecting the SourceTree performances
Cause 1
Low space on disk
The most common cause of this problem is that you are low on disk space. If your hard disk is 90% full or higher, OS X begins to have some performance issues in general, but it especially affects the overhead required to launch new processes. Since SourceTree relies on launching Git/Mercurial (hg) and other processes regularly, running out of disk space adversely affects it.
SSD drives seem to be particularly affected.
Cause 2
Repository run on a remote drive
Another possible cause is that you're running the repository on a remote drive, such as a NAS. This is not a recommended configuration, and it can actually cause other issues, such as corruption in Git if you're using a Windows (SMB) share (note that this is a problem related to Git and not to SourceTree).
Resolution
Resolution for Cause 1
Free enough space on disk
Clean up your hard disk so that you're consistently running at below 90% utilisation
Resolution for Cause 2
Repository run on a local drive
Move your repositories so that it runs on a local drive