Had the same frustrating problem in low light the other day... (Canon 24-70 at F2.8, 1/125, ISO 1000) Even with a SpeedlLite 600EX...
My 1D IV was super fast to focus. The 1DX takes too long.
I was, though, using The 5 surrounding points... I switched to single Point next day but I haven't tested low light situations yet.
I agree with most of the comments. Can't let the camera decide. Just place the single focus point where you need it.
One problem I found with the focusing points is that if you are vertically shooting a single person, standing up, full body and filling most of the frame, the closest AF point is under the neck. Usually I try to focus on the eyes or face... Somehow some of the AF modes have different "reaches" Some are closer to the edges than others...
I'll keep researching.
Good luck to all!