A few years ago when I upgraded to the 5d3 and 600ex-rt flashes I was frustrated when I found that the AF assist beam caused the camera to slow when it acquired focus. I started a post about it and a few others had the same issue. Canon issued a firmware update and the problem was solved. You can look that the thread here:
http://www.canonrumors.com/forum/index.php?topic=10275.0
I received the 1dx2 in May and have shot several weddings and receptions with it so far. During each reception I have shot, I have been a little thrown off with the AF speed with the 600ex and AF assist enabled. It is just a hair slower. It is nothing like the previous issue I had with the 5d3 and the 600ex, but it is still an issue for me. I have tuned it on and off several times and every time it is off, the focus is instantaneous and when it is on it hesitates a little. It will find focus instantly, but it doesn't allow me to fully press the shutter release instantly, which bothers me. I end up having to wait that extra moment and I don't like it. I know I can further prioritize the release of the shutter over the focus, but then I run the chance of getting slightly out of focus shots. I am curious if anyone can confirm if they are having the same issues. Again, it is ever so slight, but it bothers me that it is an issue at all. The AF assist should make the AF faster, not slower.
http://www.canonrumors.com/forum/index.php?topic=10275.0
I received the 1dx2 in May and have shot several weddings and receptions with it so far. During each reception I have shot, I have been a little thrown off with the AF speed with the 600ex and AF assist enabled. It is just a hair slower. It is nothing like the previous issue I had with the 5d3 and the 600ex, but it is still an issue for me. I have tuned it on and off several times and every time it is off, the focus is instantaneous and when it is on it hesitates a little. It will find focus instantly, but it doesn't allow me to fully press the shutter release instantly, which bothers me. I end up having to wait that extra moment and I don't like it. I know I can further prioritize the release of the shutter over the focus, but then I run the chance of getting slightly out of focus shots. I am curious if anyone can confirm if they are having the same issues. Again, it is ever so slight, but it bothers me that it is an issue at all. The AF assist should make the AF faster, not slower.