I did notice those problems with my 5D3 and a 580EXII flash. It just takes too long to focus.
I tried 580EXII with my 7D and noticed that the assist beam was bigger, lighter.. well, there is really a difference in the beam.
I think if you try to focus on further (and by further I mean about 3 meters) the assist beam it's not so effective. I say this because I tried to focus on a close object in an almost dark environment and it was ok.