Generally speaking sp2 is a very big collection of drivers, securuty and other updates and additions. It can heavilly influence older computers both in performance and compatability. So much so that I have seen computers with processors in the range of 1 Gb that slowed down or froze to such an extent that I had to remove sp2 and shut down windows updates. I consider sp2 and the other updates as very important and wouldn't recommend this step but in extreme cases.
Looks to me that you have somewhat of a similar effect. You can temporarilly uninstall sp2 through add/remove programs in control pannel. I do not recommend this because the uninstallation can end up messing other things and because of the importance of sp2. However, I made this a number of times, in most cases without ill effects. If you are willing to try - go ahead. Whatever the case, even If you find that it improves boot-up time I would reinstall it again.
Before taking any steps with sp2 please disable all antimalware programs that run in the background.