User blog comment:Rtyh-12/1.1 framerate issues - Help!/@comment-80.162.250.181-20111202192616

I have had the exact issue you are describing. The fix was to Delete the Local Files through steam and re-install! (Do not worry, it will keep your characters and worlds etc.)

Alt+tabbing while in Fullscreen will also mess stuff up, so if you are doing other stuff than terraria, play in a window and maximize!!!