I think it is related to this thread:
https://www.linuxquestions.org/quest...es-4175628845/
I still don't think your problem is related to XDMCP. Please check your resources and give more accurate information about your environment (and obviously your resource usage).
what kind of thin clients you have, what kind of server you have and why do you think your problem is related to XDMCP. How did you find it out?
From the other hand you may try VNC or local login on thin client with ssh and also there can be other possibilities, but without knowing your needs and your configuration hard find the best solution.