Such slowdowns might be caused in my experience also by other possible hardware issues, battery included. You might try disconnecting the battery and run off the mains only..if that solves your issue you have your culprit.
Other sensors failures might throttle the CPU too..if you have an external bootable drive, try booting from that after disconnecting the internal hard disk cable and see what happens..if slowdown persists, you can rule out a cable failure and concentrate on other possible causes.