First make sure that
-your usb hub is a USB 2.0 hub.
-the usb port on your g5 that you plug the hub into is a usb 2.0 port.
you can also try plugging your devices directly into your g5 and testing the speed.
depending on what you have plugged into your hub, you could be maxing out the bus with other devices (ie. speakers, hds, etc.). Just remember that other devices will be using a part of the total speed.