The problem is the lens, not the camera. I have found that my 135 f/2 will not focus well in low light no matter what 1 series camera I use it with. My 70-200 f/2.8 IS II is way better than my 135. I shoot dog sports in very low light and the 1Dx works perfectly at 12 fps in light so low you cant read the buttons but it only works this well with one lens - the Canon 200 f/2. Yes, I understand the price tag, but if you want f/2 and fast accurate focusing then this is the lens to use. It is twice as accurate as my 70-200 so you can imagine how much better it is than the 135 f/2.
Here is a shot in very, very low light (we refuse to shoot there now) where the 1Dx with a 200 f/2 hits the focus without issue. Sorry this is a processed image but the original looks very good and is sharp. The high ISO softens it a bit, but the auto focus hit perfectly and the numbers on the stickers on the shirt were sharp. I actually added grain to this processed image to match the retro color processing. Note: that dog is super fast and runs 7 yards per second!
1DX, 1/800s @ 200mm f/2, ISO 25600, TV mode, +2/3ev, auto ISO, focus mode 4, single point + 4 expansion, using 2 down from center point (x type), 12 fps.