Hi John Bremner,
Welcome to Microsoft Q&A forum. I'm happy to help.
You may try a workaround from user AJ42 on this thread: Windows 11 update KB5074109 reportedly reduces gaming performance.
Disclaimer: This is a non-Microsoft website. The page appears to be providing accurate and safe information. Watch out for ads on the site that may advertise products frequently classified as PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.
As this is only a workaround, it may or may not work on your system. If the workaround cannot help, please uninstall the update and wait for news from Microsoft.
I hope that above information is helpful to you.