You may have some excess antivirus activity in opera cache folder. If your Opera slows down again, turn antivirus off (just for testing).
Also, some content indexers (Windows search indexer, then some media catalog or CD burning programs tend to run background indexers too) may choke on Opera cache folder.
Does your HDD light blink while Opera slows down?
Another possibility is that all (most) your open tabs contain flash or other animations, running on background - but such situation should affect other browsers too.