Before I rooted/flashed I noticed that would happen when using the back button out of an app, back to the home screen. Seems like it took a laggy second to show the homescreen and redraw all of the icons. What's odd is when I first got the phone, I don't remember those issues. Maybe I overloaded it with apps over a few weeks.
I'm running CM10 now and it's fast as ever, buttery-smooth. None of the lag I had on stock Samsung ROM.