Have you defragged? SP2 installation causes quite a bit of fragmentation.
SP2 will replace a lot of system files; even if that doesn't
invalidate the prefetch data, it will skew the OS's sense of what
files are "most often used".
So a defrag straight after SP2 will help, but only partially.
After a few days of use, new prefetch data will be built to more
effectively guide Defrag in moving file content so that the files that
really are most often used, will load faster.
So a defrag a week after SP2 may help a lot more ;-)
In my case, I saw no adverse performance hit - but then, I build to
limit exposure to this factor! C: is under 8G and is FAT32, so no
matter how fragged it gets, the most-often-used files are always
within the first 7% head travel on this 120G HD.
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Never turn your back on an installer program