I've just read the last Windows XP SP1 vs SP2 Performance comparison published today by Short-Media.
It's an interesting comparison focused on performance comparison on various fields, and the official results are that "Windows XP Professional with only Service Pack 1 installed is faster 2/3 of the time... The test PC equipped with Windows XP Professional Service Pack 1 was an average of 0.5% faster than the same hardware with Service Pack 2 installed. The percentage difference between faster and slower is insignificantly small".
Personally, I can't observ these results on my reality...
I've installed XP SP2 on 3 different machines (on my desktop P4 3Ghz with 1Gb Ram, on my laptop P4 2,6Ghz with 512Mb Ram and on an Athlon XP at work) and all the machines have a terrible slow down on performances (plus other problems on some cases). Windows XP SP1 has (for me) better performances than XP SP2 and my personal machines are (at the moment) not upgraded to the last Service Pack.