Don't think it's what you've downloaded in addition to the game, because I've downloaded plenty of things and all of my presets have ran smoothly in-game. What it could be is if your preset has glowing/moving markings or special coding for special effects, but if not then I would recommend lowering your distance, turning off water reflection, and perhaps changing how your game runs. Perhaps try switching from OpenGL (if you use it) to Direct3D9.