Loosing 50% is a little disturbing and understand the frustration. Part of the fun crunching is getting over the hiccups that occur from time to time. Good suggestions have been proposed. If your willing, post the information requested so others can help find a solution. Couple things come to mind:
What has changed in the past year on your computer?
All fans working?
Clean it out lately?
Other programs running slower than normal?
Open up activity monitor to insure all cores are showing correctly?
Page file growing enormously?
Installed ram showing correctly?
Just trying to figure out if its a software or hardware issue. Let us know.