The USB port speed is constant so if my system is fine upon a fresh installation then USB speed is irrelevant to the problem. If it were a problem then it would manifest itself from the get-go.
Good point, Jon. I have all my project files on an external 1TB Lacie HD connected to a USB2 port. Premiere and OS are on my internal HD. Not sure where my cache files are, can you tell me where to look?