Make sure you have exited not disabled your antivirus program BEFORE you install sp2. A lot of people don't and it leads to problems. Also set a restore point BEFORE installing sp2. Gives you a chance to roll back to the previous setup if you do encounter problems.
As for slowing things down, sp2 is rather large and does more. SP2 is a rewrite of the core code that makes up XP. You can still go into the services and streamline what is not needed by you for the way you use your computer. Check this site out for the do and do nots in editing the services.
This site is trusted by many of us old timers as being a source of reliable information.
Hope this helps.