Here some feedback:
The server was ATOM 2 cores 2 threads.
I had to edit on server side node.cfg to test each modes: (h264, vp8, mjpeg) then i uncommented EnableDisplayServerVideoCodec 0, DisplayServerThreads auto, DisplayEncoderThreads auto.
Lowering the resolution on NoMachine help a lot if you stay on h264.
In my configuration, vp8 encoder is lighter and hold up fairly well, you don’t see the processes nxcodec.bin .
I tested on a better processor and no issue at all, so yes h264 encoding need a lot of cpu usage.
And about firefox-esr, it still ask a lot of cpu when scrolling but it s not about nomachine.