Suddenly our jetty is running extremely slow.
As we look at the jmx console (see screenshot), we see that the complete pool is allocated in very short time.
What could be the problem here?
I think this is a question that would be better off addressed to the jetty user community mailing list at [hidden email], as there's no indication of a jetty bug in your post. Your server may have had a spike of requests, or slow network/clients causing more threads in the pool to be activated. You'd need to dig into this a bit more, looking at request logs, network logs, thread dumps, turning up debug logging levels and using a network packet sniffer it if happens again.
If you can find evidence of a jetty fault (eg no request spike, no slow clients etc) then please post the full details to the jetty issue tracker which has been over at Eclipse for a couple of years now: https://bugs.eclipse.org/bugs/buglist.cgi?query_format=advanced&product=Jetty&list_id=4122431