The Windows 10 cumulative updates KB5001330 and KB5001337 came out earlier this month, containing various security fixes. They also messed with some users' in-game performance, causing stuttering and FPS drops as well as the occasional blue screen of death. March's KB5000842 preview update had led to similar problems, but was less widely noticed due to being an optional install. In both cases the advice was the same, even from Nvidia's staff: just roll back the update.
Microsoft is now aware of the issue, saying that, "A small subset of users have reported lower than expected performance in games after installing this update. Most users affected by this issue are running games full screen or borderless windowed modes and using two or more monitors." Its solution is to use Known Issue Rollback, "a new capability that can quickly return an impacted device back to productive use if an issue arises during a Windows update." Basically, over the course of today Windows 10 should undo the problematic change automatically.
And then everything will be hunky-dory. Until next time.
If you ever need to get rid of a Windows 10 update manually, head to Settings > Update & Security > Windows Update and click on View update history. From there, click on Uninstall an update, highlight the one that's causing trouble, and click the Uninstall button.