For the past two weeks my 360 has been having a lot of issues with slowing way down beyond an acceptable level. Such as the console taking 3-4min to turn on, then another 2-3min for the dash to pop up, and then 3-5min for a game, Netflix or any other app to start up. I first noticed it while playing through the GoW2 campaign as the load times after dieing got to be longer and longer with each successive one, and after some fiddling around (disconnecting and reconnecting the hard drive, clearing the cache, deleting unnecessary saves and games I no longer need) I found the cause of it to be the GoW2 install. Upon deleting it everything went back to normal and ran as it always did. However, any games new games I've installed since have had the same effect as well as arcade games I've downloaded.
Clearing the cache does nothing, being connected or disconnecting from Live has no effect, but the system runs normally when the hard drive isn't connected.