there are limitations to how fast a TCP connection can go with a certain round trip time, the default TCP settings in windows aren't optimal for fast connections, however it shouldn't cause it to go that slowly, and not 100% of the time, if that were the cause it would be a different speed for different users
you can try adjusting the TP settings on your system to make it send more but the effect of it only being changed one end is limited a little (some things are negotiated between the 2 ends, so goes with the lowest settings... which would be the default settings the other end is using) but wouldn't hurt to try
if you do want to try changing your TCP settings, go to start>run and type "regedit" and click OK, then navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters and look for the TcpWindowSize setting, and increase it, try setting it to about double whatever it is currently set to, this is assuming all other settings are default and you haven't ever run one of those many optimizers available (which can easily cause reduced speeds if used incorrectly)