I was running a google meet call in Firefox and had a few other tabs open, including a gitlab one with an embedded video (which might have been part of the cause of this) when I noticed considerable lagging in the responsiveness of windows, not just firefox. I found the following conspicuous messages in the console app:
IOGraphicsAccelerator::newUserClient(): process plugin-container (PID 1934) has too many contexts (510) + queues (0) + device (171) + shared (171) created. Possibly leaking?
It also affects opening browser tabs in e.g. Safari and general system responsiveness.
I couldn't find much on this kind of error message other than the plugin-container process seems to belong to Firefox.
This is a Macbook Pro with macOS Catalina 10.15.7.