Secret functionA cache clear function has been dicovered on the Xbox live dashboard.
Go to the system blade and then select memory. Press Y on the HD symbol and then press X,X,Left Bumper, Right Bumper, X,X. Once you do this a messsage will appear saying: Do you want to perform maintenance on your Xbox 360 storage devices?
Performing this function will also clear any software updates. It's unclear whether this will cause any problems to your hard drive - but I've tried it and everything seems fine.
I tested it out on Oblivion when I was experiencing slowdown and, lo and behold, the cache clear appeared to rectify the problem.