Out of curiosity I ran a GWT compile of the same app on a Lenovo T510 with Ubuntu 12.04 , and then on Windows 7.
Here are the results after 3 GWT compiles of the same app, in succession:
1. Ubuntu + Eclipse + GWT2.4... 65 + 56 + 48 = 169 secs
2. Windows7 + Eclipse + GWT2.4... 80 + 70 + 72 = 222 secs
Compile time was 31% longer on Windows 7, using the same hardware and code... I did not expect such a big difference.