Updated April 17.  Take it with a grain of salt. See details at the end of the post. 

We recently switched from using Subversion to Git for source controll management.  If you're not already familiar with Git you should go check out the web site.  It's some great software.  There are plenty of places with big lists of why Git is great, so I won't repeat them all.  Suffice it to say that it's fast and friendly to the developer. Also, it can super charge your IDE.  Seriously.

As an unexpected benefit of moving to Git I discovered that it really sped up my IDE.  I've been using RadRails from Aptana (which is awsome... err, rad) and had started to notice it getting sluggish.  I assumed this was just because I had several projects open and that they were beginning to get large.  After adding my new Git based working directories to the workspace and removing the old Subversion based ones I discoverd that RadRails was back to it's old snappy self.  Hooray! 

I'm guessing that the speed increase is related to the drastically reduced file count/size of the working directory.  Just for reference, my old Subversion based working directory for one project was 1.7GB (1,700MB) 95MB.  The new Git based working directory is only about 41M.  If I take the Git version and delete the .git directory (where all the magic happens) I get a file size of 28MB.  Let's see that in table form.

Wow! One more reason to love Git.

Update: April 17

OK, I'm a doof!  For my original numbers, the Subversion directory happened to be holding a lot of end user data (mine, from testing) that is not an actual part of the project.  After clearing that out and doing a fresh checout from Subersion it shows that a Subersion based copy of my project is 95MB, not 1.7GB.  I did go back to see how that impacts the RadRails performance, and it does still seem like the Git based project is a little speedier, but it's not a pronounced as before.  YMMV