When there is little work to do, the cpu will downclock itself to save energy and heat.
Run prime95, and you should see all cores running at the max multiplier.
As to running slow:
1) Check that you do not have a virus or other malware running.
2) Check your temperatures for cpu and gpu. If they get too hot, they will downclock to protect themselves.
3) Do you have enough ram? 4gb at least. If you are a heavy multitasker, you might need more to prevent conflicts with games.