click here explains what Netframework is. Version 3.5 is the latest version - and SP2 is the latest update to it.
"Coincidence?" - No - it probably has slowed your system.
It may be that your system is already overloaded or slowish, or old or low on resources - and this update has "pushed it over the edge".
Try approaching the issue as just one of you system running slow, and look at clearing it out and/or upgrading it.
Sorry but it's difficult to be more specific as you haven't told us anything about your system.