Many possible things. Log file please?
Possibilities:
1. Trying for too high a frame rate -- asking for 60 fps when you can only give 30 is a problem.
2. Trying for too high a quality/too slow a speed -- asking for "normal" when you can only support "very fast"
3. Other things happening on your system.
4. Minecraft's GC settings making the GC use too much CPU (I am not kidding). (NB: Not a factor in vanilla, but can be in modded).