You are right that the 1Dx when set to 10fps or slower does track and lock focus better than when it is set to 12fps, however there is still something strange going on, because in my testing in the SAME lighting/contrast conditions where the 1Dx set to 10fps or slower in AI-SERVO fails completely (meaning not slow focus, but can't focus at all), my older 5D2, 5D, and 7D in AI-SERVO mode all focus lock and track just fine. If my 1Dx set to 10fps or slower focused in AI-SERVO as well as those older cameras then I would be totally okay with this and move on, but as it stand that is not the case.
I think you are right in that there must be some sort of engineering tradeoff going on here, but why is it that in the same light that the 1Dx (at 10fps or slower) totally fails, the older cameras focus and track just fine? This is what I would like for Canon to either explain or fix if possible.
Otherwise, I agree with you that this is the finest camera I have ever owned.
Excellent questions and I wont pretend to have all of the answers. Perhaps Canon has made a trade-off for AI Servo tracking motion sensitivity compared to stationary subjects, in low light, low contrast. I don't think that this is actually anything new as my MKIV behaves the same as the 1DX. When I locked in with the 7D in AI Servo, it did take a couple of seconds and it was not nearly as fast as initial One Shot focus lock. So even if it does this one thing better, the MKIV and 1DX are so much better. If I moved the camera around a bit on the target, acquistion was a little faster.
I have never had any trouble auto-focsuing with the MKIV at sports venues, and the MKIV performed exactly the same as the 1DX when I did the low contrast tests. Now in better light the 1DX focuses faster than the MKIV. Look, you could own a Ferrrari and if you're driving it in the rain, and at night, then you will need to keep your foot lighter on the pedal. However, with some dry roads and daylight then you'll be in heaven.
Canon states in their Manual that in low contrast, and low light you may need to use Manual focus, which sounds ridiculous.
For initial focus acquisiton, other tehniques would include, grabbing the edge of the uniform, helmet, transition from jersey top to pants, alternating from one player to another, switch from background to foreground, pump the One Shot button, lower the FPS, change to release priority and then wait for the player to move a certain distance which definitely aides AI Servo in tracking mode, you can also turn the manual focus dial. Now, I don't actually regularly use these techniques, but they are all available to us. I also do not use all AF points selected, unless I'm shooting into a clear sky. I usually use manual single point AF with expansion points, so I put that point right near an edge if need be, but like I mentioned earlier I haven't found it to be even noticable in real world shooting situations, however that does not imply that some cameras may be having some difficulties.
My friend has a 1DMKIII and he shoots Surfing in One Shot Continuous High speed shooting, because he says that its better than his AI Servo mode.
I think we are all going to have a slight learning curve with all of these new settings, and I can't wait to play around with the new AF Cases and tracking acceleration and deceleration sensitivites in the new AF menus. I haven't had a chance to experiment with this yet, maybe it will help.
If Canon can improve on this with a firmeware update then I would be thrilled.