MC 2012.9 slows right down (to usless) in Colour Correction mode

I have a 5 minute corporate that I'm working on. It is a 1080p project, but some of the interview footage was shot at 4K (to give re-framing scope).

I have been doing some basic CC work, but MC slows down to a snail's pace, often hanging for minutes at a time before it comes back. Swirch back to SourceRec mode and MC is back to normal speed.

Anyone else seeing this? It's driving me nuts!

I'm on 'edit 1' of the systems in my specs - basically Win10 Pro (1903), Core i9 (10 core HT), 64GB ram, Quadro P4000 driver 441.12.

Any suggestions appreciated

MattB