A
Full Speed rate of 12 Mbit/s (1.4
MiB/s). Full Speed was the fastest rate before the USB 2.0 specification and many devices fall back to Full Speed. Full Speed devices divide the USB bandwidth between them in a first-come first-served basis and it is not uncommon to run out of bandwidth with several isochronous devices. All USB Hubs support Full Speed.
Taken from Wikipedia, so if youre using USB for your Modem and not getting full speeds, each USB hub on your system has 12MBit shared between the ports so perhaps try changing the port your Modem is plugged into to one that is next to an empty one or next to a low bandwidth consumption one such as a mouse.