I'm having the same situation, except mine is rooted. I've been monitoring my phones performance avidly lately to see if there's any other services or apps that could be removed to eliminate this problem. The little bit that I could gather with my limited knowing and ability to delve into the phone more is that the whole slew of GO apps, services, & add-ons are the culprate. They take up massive amounts of memory thus hindering the devices performance. The whole series of GO apps are demanding.
For GO Launcher Ex specifically though you can increase performance by using the default UI theme. The downloaded themes use up a bit more then the default UI. Don't forget that you can reboot the GO Launcher from the options button menu. Rebooting the launcher does a great job of clearing up space.
Overall though, I think the best thing to do is to just ditch as many of the extra GO add-ons. I find them to be bloatware for the most part.
What other apps and services from GO do you have though?