Running really slow

My system has suddenly started running really slowly (even typing this into Safari under MacOS the letters finished coming up 15 seconds after I finished typing!) but now I've shut down Windows 10 everything is fine again. The Win 10 Window is still up BTW.
Running on a 2013 MacBook Pro, latest versions of Parallels, Win 10 and Mac OS. Sometimes plugged into a 2nd monitor (IIyama 4k) The system has been running fine for ages, but when I came in this morning and restarted it, suddenly everything went to pot. Possibly a Windows Update thing? It did say it was doing something to do with updates the other day. Please help - I have to do my work on Windows and it's pretty much impossible now.
<Update> If I stop using the 2nd screen, the speed issues disappear. Also, the MacOS Activity Monitor says Windows is back to using something like 20% of the processor rather than over 200% as it can be with the 2nd monitor attached.

 

Further to this, I realised that the only thing I'd changed was plugging into my extra screen when I got into work. I've unplugged it and things work fine speed-wise, it's just hard to work on massive spreadsheets on a 15 inch screen.

 

Hello @WillD1,
You could try reinstalling the Parallels Desktop application. Use this uninstaller tool to remove Parallels Desktop.
Also try the solution from this article and check if the issue still persists.