I have a 60D and 580EX II, and I have the same problem (focus takes 2-3 seconds and often misses with AF assist). I don't understand why this happens, but judging by the comments here it's a common issue for Canon. The "fix" is fairly simple: disable the AF assist light and aim for high contrast areas on your subject. Even with the 60D's more basic AF, I've never had a serious problem in dark halls, as long as I pick good spots. Was hoping they would fix this for the 5D3 and 600ex-rt, but sounds like we are still stuck using non-assisted AF. Since I don't have a firmware update to fix this, I assume it won't happen with the 5D3.