What is needed is some kind of "wide area network optimizer" (search for that). Basically, you need the windows machines on your network (on either end) to connect to a way-station where they have low RTT and then the WAN optimizer is some kind of network router with a very large buffer so that they can support wide data windows over the long RTT connection. This can be done on the cheap with a Linux box and some tunneling, but there are also commercial devices. They are popular with companies that use satellite (looong RTT) connections for internet connections.

There is no "fix" in Windows itself.