My Projekt runs on Ubuntu 20.04 and Python 3.10.9 with Plone 6.0-latest (Classic-UI) - nginx/varnish/haproxy with 4 zeoclients - default wsgi threads 4.
After some content modifications the zeoclient(s) starts to have a CPU usage > 100%
I dumped the threads of such a zeoclients with py-spy and got this:
Any ideas whats going on here?
Yes, we also see that and reported it here:
and here:
The workaround that worked for us was to set:
[instance]
http-fast-listen = off
Let us know if this fixes your issue as well 
Thanks! Looks good so far ... running now for an hour without any CPU issues. I'll keep an eye on that.
After one day running I can report, that there are no more clients with 100% CPU usage. I'll mark this as solved.
But maybe Zope should change the code as the waitress developer outlined?
Yes ... marked as "Workaround" 
hum ... today it popped up again ... I've commented here 100% CPU on all instances when http-fast-listen is enabled · Issue #188 · plone/plone.recipe.zope2instance · GitHub ... maybe there are more hints on this.
After some debugging sessions and hints (see the github issue 100% CPU on all instances when http-fast-listen is enabled · Issue #188 · plone/plone.recipe.zope2instance · GitHub) I ended up getting rid of waitress and trying pyruvate WSGI Server (@tschorr is the author) using this little recipe examples/plone52 · main · Thomas Schorr / pyruvate · GitLab which also works for the latest Plone 6/py3.11.