Oscillating CPU load while rendering video

New TM user, just learning and making the best of my older PC with intel quad core CPU and RTX 3060 GPU

Video export is very slow, as expected, but I noticed that CPU load oscillates wildly and only averages about 7%. Resource monitor indicates that there is a TM process not responding.

Screen shots below. Can anyone explain what’s going on here?

I could upgrade my CPU to an 8 core with similar clock speeds, but is there any point?

Any help gratefully received, thank you.