The computer is still running very slow when I use the cellular modem thru a USB port.
Sorry mate for this question but why you are thinking this is a USB driver problem.
As I understand the modem has been recognized by notebook. this means that USB ports are ok!
You issue should be called: notebook run slow if cellular modem is connected.
I also tried running the cellular modem with Ubuntu 12.04 (running from a USB stik), but that is also very slow. So I think there's some deeper problem in the computer.
Well you are right… there is other issue but not with USB drivers…
One question: what do you mean by: running slow?
Do you mean the data throughput ?
Using modem you will not get fast internet connection like using WLan or LAN