Benjamin,
I've written a file update routine for my game using Multisync and the code looks pretty efficient. Once it was all done and working I plugged in a few lines of code to see what the transfer rate was, and to my surprise it capped out at 3 K bits per second.
I also noticed that on the client end CPU usage was pegged at 100%. The server is running about 17-19%.
I've tried a few more things to optimize my code, but it's not making any difference. Have you done any performance tests in the past and found this to be about the best speed that Multisync can do?