can you run something like iperf3 or openspeedtest between the server and client to prove its a network throughput issue?
do you have a network switch you can add to avoid switching through your router (if it is indeed bad?)
Have you ensured you arent unknowingly using wifi at either end?
I’m just now realising that Nukumizu’s name has mizu (water) in it, and looking up the nuku part can mean “to extract” … and they guy’s hobby is water