@Pierre-NicolasL
The solution for me was quite simple (although parallels haven't recognised it as a defect yet). Do as in my previous post (I followed what Paul16 has suggested and his comprehensive 'how to', thanks Paul16):
Got rid of Access Windows folder from Mac and Win 10 CPU usage 2% (yes two percent) at idle and MacOS Sierra CPU usage at 9.4%. That is a staggering difference.
That is the culprit (or at least was in my case). The system seems to be thrashing constantly trying to see if anything has changed in the Windows world. Give that a try.
Cheers,
Kevin