I'm wondering if USB performance of a babyface or fireface UC will change based on how many usb devices you currently use with your computer. 

I have a Mac Pro, and the rear USB ports are loaded with 2 hubs filled with usb keys, mice, keyboards (ascii and musical) etc., etc.  If I connected an interface to one of these back ports would performance suffer due to overuse?  Or will there be no change?  Also, are the front and rear ports separate in terms of performance?  I could potentially only use one front port for the interface and the 3 rear would be loaded up with the rest of my devices.