So I've been doing some further testing. I can create a situation on my mk3 and 600 ex-rt where I can notice a much less snappy AF lock. I was not able to re-create the lag when trying to focus in a very dark (almost black) setting. Maybe, that's where I messed up in my prior testing. Maybe a completely black environment creates enough contrast between the environment and the red focus assist beam to allow focus whereas a dimly lit situation is lit just enough to give the camera trouble focusing because it can't really see the red AF assist beam as well.
I'm able to notice a significant lag when the ambient light is dim (proper exposure in camera at f/2.8, 1/50, ISO 5000).
Seems to me like I would just turn the beam OFF when I notice the ambient light is about at this level. Am I still over simplifying the issue? I get that pros need their focus to be snappy to catch those moments. Wouldn't that mean a few test shots in the environment first to gauge the settings required....including wether or not to turn the AF assist beam on/off?
Maybe I don't understand the depth of the issue because I'm not coming from a 5D mk2 where the focus is supposed to be faster. That said, I'm an engineer by trade. We are trouble shooters by nature. It seems like there's a pretty easy fix for this (just based off of what I've been able to test). If this really is an issue for a shooter, just turn the beam off when you feel like it's slowing you down. Does ambient light in a reception hall or your venues change constantly for you guys? Isn't this just a matter of your experience telling you how your camera should be set to capture moments? I'm not trying to be condescending. promise. I still don't understand what the big deal is.
You are right the issue is occurs in very dim but not supper dark situations. I shoot wedding receptions all of the time and the lighting changes constantly. It happens during grand entrances and first dances all of the time. You get set up for certain light and all of the sudden lights go off completely. I always ask in advance to try to plan for those instances but they often happen without warning. Different parts of a reception hall are also lit differently or facing different ways. For example shooting with or against the DJ's lights. I switch it on and off all of the time and have it registered on my menue settings so I can get to it quickly. The problem is it still takes 5-10 seconds to do that and there are times you can't waste that time. The bride and groom in a grand entrance and you loose 5-10 seconds and you may miss everything or most of it. It is a pain to have to switch it on and off and I can't figure out why I need to. If Canon gave me a reason then I could live with it but they don't even acknowledge there is a problem.