Sounds like there are a lot more problems with networking your computers together than just frame rate! Does each of the computers have the same aircraft, scenery, etc installed? It would be interesting to see the log.txt from the slaved computer when it is running the flight on its own and has good FPS, and then a copy of the log from after using the network set up with that same flight on the master machine that makes it go crazy.
Otherwise, I would probably need much more detailed information about this situation to investigate further. I'd need you to outline every step you take to see this, so that I could try to set it up _exactly_ the same way on my machine.