My experience has been that SP3 makes systems run faster, rather than slower. Of course, that's AFTER a few reboots with Windows running some type of additional updates. In some rare cases, I've had random, small, third-party programs slow down the system after SP3 but it's always been an issue with that specific program and not SP3. It's never been a critical program, so I've considered SP3 to be OK.
If your systems are running slow (or not at all), I would start by looking at the auto-start apps and isolate which one may be causing the slowdown. I've had things like anti-virus software, that likes to start FIRST, conflict with what Windows wants to do with SP3 (not sure why). In that particular instance, turning that option off and then on again resolved it.
SP3 is major enough that normal drive maintenance is a given (cleaning TEMP files, defrag, etc.). Also, as usual, the more RAM you have in the system the better your performance will be. On most of our systems, 512MB is bare minimum. 1GB seems to be sweet spot unless you're running a lot of apps on startup. Hope this helps.