Thank you for your reply.
The the RAM was running at 50% throughout the session.
The significant observation was that the GPU throttled back from 100%. The GPU management system was set to “turbo - game”, as setting the GPU management system does give a similar result if set to “office - quiet fan” mode.
The Max out of the GPU does not bother me as it supplies an appropriate frame rate. What bothers me is the throttling back of the GPU to 20%. i have the latest drivers, and Nvidia manage this on my behalf.
I note that Zilla076469 stated that when the game was restarted, the FPS was back to normal. I found this too. Like Zilla076469, I also found that resetting the graphics quality from anything from low-end to ultra, made no difference to the low frame rate, once low frame rate was established.