Tore, we had a similar challenge and we managed to get higher throughput with the help of 16 worker queues processing each 100 req. Refer to this thread for more details.
It also depends on where you are running the CRON jobs (queue workers) to process the queues. If you are having it in the same machine as the DB server, it would have an impact on the mysql processing speed, rather it is recommended to have a separate server only for the cron jobs processing.