In addition to getting rid of your pix in memory and deleting unused apps, I also highly recommend that you go into Manage Apps and select CLEAR CACHE (Not "Clear Data") for EVERY app it's available (lit up) for.
My GF's Nexus S 4G had your same issue and I found the Cache tip when researching the issue. It fixed the issue as well as making the phone run faster. I did the same to my Galaxy S2 with similar results (regarding speed improvements).
Give it a try, as you literally have nothing to lose. As long as you only clear the cache and NOT "Data", you'll not lose any of you existing login settings, high scores, etc.
Good Luck!