The focusing is a bit slower than the L lenses I've used (24-105, 135, 17-40, rented 14L, 24-70L v1, 70-200 2.8 IS v2). Not bad really, but not snappy like I'm used to. I shot most of the night in AI-Servo, since they were dancing and moving all around. As I said the lighting was quite challenging, but even when the AF points I was using was over the subject in decent lighting, I always felt like I had to wait a second or two for the camera to lock focus and start tracking. Quite annoying actually, and nothing something I'm used to. Part of it may have been, as stated above, my fingers rested right on the focusing ring so I may have confused it.
Leaving aside the IQ, if I can't figure out the AF in this kind of lower light, I might have to not get this lens, which would make me sad as this is exactly the environment I'd use this in. I'll try using my 24-105 briefly in the same lighting conditions to see if it shows similar hesitation, and if so it's the camera/lighting, and not the lens and I'll have to test out in other dim lighting conditions.