When 200 builds are queued, all other builds wait even if there are no currently running builds. This happended due to the PPC64 temporary block and all builds (PPC64, x86, x86_64) were blocked. I solved temporally removing some of my jobs that had ppc64 builds to do.
This is intentionally due some other behaviour of OpenStack which affect us, where you can not have more than 200 VM per one tenant. The long PPC queue was caused by some other issue, which was resolved and should not happen again. So the real issue is that "when there is more than 200 task claiming to be running, no other task is executed" and that is and will be intentional. Such situation need manual intervention anyway.