Does changing the theme improve system performance?

Solution 1:

It is certainly possible, some GTK engines/themes render faster than others.

You can test this yourself using the benchmarking software GtkPerf.

1. Install the package gtkperf

   sudo apt-get install gtkperf
   

2. Open it Applications ➜ System Tools ➜ GtkPerf:

   

3. Click Start and sit back while it does its tests:

   

   The results will be listed in the first tab.

4. Switch to a different theme and then run it again. Compare the results with the first theme. Personally, I ran this test on Elementary and Ambiance, and the former ran the test about 2 seconds faster, your results may vary.

Solution 2:

The theme does change the redraw rate because a theme could have (maybe) a blurred background-frame (like in Windows Vista). This would require a lot more CPU. Maybe it might have a gradient for a background and it would have to redraw that every-time you move or resize a window, change it's content. I'm not 100% sure but I believe that would be the cause.