Do you think the cpu’s server cannot handle encoding h264 and could be the reason of high cpu on on a mac client ?
This is possible if you say that the resolution is very high and given that the CPU is not very powerful. But also note that such CPU usage just means that a bit more than 2 CPU cores are used of the 8 at disposal. To limit CPU usage further, besides reducing the resolution, you can try to lower the frame rate in the Display settings panel.