12 seconds? This is insane.
Have you considered using celery?
Never used redis-rq, but from what I see based on docs it is not really good for big numbers of workers
Redis queue usualy based on BLPOP command, which can work with multiple clients, but who knows how much it can really handle for one key.
So I suggest you to switch to Celery or writing your own tasks distributor for python-rq, which wont be easier then switching