I already had 3D acceleration enabled, but had only allocate 1-CPU. (The host has 4 CPU’s, 8 threads). I doubled this to 2 CPU’s, and the utilisation on the guest dropped by half, which would make sense, since there’s now twice the grunt. The total CPU on the guest now averages around 35% when idle, each of the 2 tasks I mentioned are now about half what they were, and there’s been a corresponding increase in the host’s cpu usage.
I guess that if I gave the guest 4 CPU’s then the total would halve again. I haven’t tried this but it would make sense.
So, I guess this is the answer: that NoMachine needs a fair amount of grunt to do what it does. Fortunately I only have 1 VM that I need to log into remotely, (actually it’s my wife’s system from her desk across the room). If there were several then the host probably wouldn’t cope.
So, I’m pretty happy with the solution. Thanks once again for the suggestion.