To minimize the use of RAM, RAS buffers the data when sending from the fast LAN to the slow RAS connection.
If you have applications that send more data than can be buffered, RAS attemps of form of flow control by not issuing NCB.RECEIVE on the session until it has enough buffer space to get incoming data.
The default buffer size for each client is 655,350 bytes. If you have enough disk space, you can increase this limit by increasing the PageFile and using Regedt32 to navigate to:
Double-click the MaxDynMem value name in the right hand pane. The default, 0x9FFF6 (655,350 bytes), can be increased to a maximum of 0xFFFFFFFF (4294967295 bytes). The minimum buffer space per client is 0x20000 (131072 bytes).
You must shutdown and restart your server for this setting to take effect.