Hi, I have made progress!
Two things helped. The first was advice to type 'driver Manager' into the mint Menu, and update anything listed there. My AMD video, graphics,ATI, and something called xorg drivers all needed updating. In fact one had never been installed properly, which surprised me because Mint has been so good at installing all the right stuff automatically till now. At first, installing these made everything worse, throwing up lots of error messages after re-booting, and there were a couple of problems. But when I booted up today, ready to do a system restore with Timeshift, I didn't need to, all seemed fine. I tested MC, and there was significant reduction in CPU, but still a bit high without yet playing anything.
The second was the prompt from Bob about auto import. Because I didn't see it running, I assumed it wasn't. But after deselecting auto -import, closing MC completely, waiting for a while then restarting, MC now runs at 50 -60 % CPU when playing audio or video. That's higher than on windows, but I am happy that it's now use-able on Linux Mint. Thank you Bob, Awesome Donkey, and the Mint Forum!
I've only tried it for a few minutes, so I hope it will continue to be a keeper on Mint cinnamon.
P.S Changing pulseaudio resample to best quality made a minor difference to sound along the way resample-method = src-sinc-best-quality