AirPort Extreme and multiple HDD's
Yes, it can be very slow this way, at least compared to other methods. You have a few throttle points: the USB connection, the USB and the wireless connection to your computer &amp; Apple TV. If you stream a movie from your iTunes Library to Apple TV while Time Machine is backing up, you are effectively routing two data streams through the USB hub, and three data streams through your wireless router.

Your best bet is to get some of the hard drives off of the AirPort Extreme's USB drive. The exact solution can vary greatly based on your exact network and computer setup. A solution could involve moving some external hard drives to plug directly into your Mac or replacing some of the drives with a NAS that can be plugged in via Gigabit Ethernet.

If you can post more specifics in your question, I can update this answer to be more specific as well. How many Macs do you have? What types? Which Macs access which drives for which purposes? Etc.