We experienced similar SFTP transfer speed caps from some of our Windows XP desktops to one of our vendors SFTP servers. The weird thing was that some Windows desktops seemed to be capped at 100KB/s for upload (averaged more like 80KB/s), and other desktops on the same LAN would get around 400KB/s uploads. After ruling out NIC hardware or drivers, we accidentally made of of the "capped" desktops upload to the SFTP site at 400KB/s. We discovered that SFTP uploads speeds increased after installing the Citrix XenApp client. After comparing the registry before and after the install, we discovered that the Citrix client modifies TCPIP and ADF parameters in the registry.
After the below reg entries were added to a XP client which was experiencing slow upload speeds, (reboot system after reg change), the client was able to upload at over 400KB/s
value are added