I want to preface this with the following: I don't want to pull a fanboy move here. The only thing I assert is that a recent OS (ie. Ubuntu 10.10) can give a considerable performance improvement (without changing the hardware) compared to an almost 10 year old OS (Windows XP).
Without further ado, compiling a large(ish) Java project on Windows XP:
real 3m16.776s user 0m2.333s sys 0m0.796s
And Ubuntu 10.10:
real 1m32.169s user 2m10.488s sys 0m12.677s
More than twice as fast! Neat!
Update: a friend just got a newer machine with better processor (Core i5 vs Core Duo) with Windows 7. The new machine with Windows 7 compiles the project in ~1m50s, so still Ubuntu seems to be the better choice.