Winmodems are just that, Windows Modems. They rely on the computer to do all the processing and they are designed for windows. The reduced rate is probably because linux is pretending to be windows, i.e. slow
. In all seriousness the problem is probably due to the overhead of data processing and the drivers are probably not the most efficient in the world.
I would seriously consider getting a hardware modem, these are pretty much garanteed to work with linux and will probably solve your problem.