I'll admit I've etched precisely two PCBs in my life and neither was etched in the last 10 years. I don't recall having to wait 20 minutes for the etchant to work though. But if, like this guy, you're stuck over a tray agitating for 20 minutes at a time his solution might appeal:
http://blog.makezine.com/archive/2009/06/pcb_etching_with_the_make_controlle.html?CMP=OTC-0D6B48984890
I thought about doing something like this once- I took apart an old playstation controller and there two motors in there with pendulums on them to make the controller vibrate. I played with the idea of mounting them on my etching container to make the whole thing sort of vibrate to agitate the solution. But then I switched to muriatic and I don't worry about agitation anymore. Except on my eyes from the fumes... heh heh.