Sometimes Spotlight indexing kicks in after a major system update. It can take a fair amount of processor usage and it also slows down your hard drive. Click the magnifying glass in the upper right and see if there's any message about your drive being indexed. If it is, then that's likely the issue. It's frustrating, but it's temporary: just leave your computer on and allow Spotlight to finish indexing. Your performance should go back to normal once it's done.
If it isn't Spotlight (or Time Machine backing up), have you tried restarting? Repairing permissions through Disk Utility?