Some of the pictures in this album are taken with the combination 1DX, EF 300 mm f/2.8L IS II USM and EF extender 2X III. You can see which they are by checking the shooting information. If the focal length is listed as 600 mm, it's actually this combination that was used.
The AF speed slows down when using an extender, but that's kind of natural, and not too much to be afraid of. It's not really focusing speed that slows down, it's the rotation speed of the focusing ring. Since a 1.4X extender will give you only half of the view, rotation speed will go down to 50%. When using a 2X extender, you only cover one quarter of the image you otherwise would, and focusing speed is reduced to 25%. But due to the magnification, the apparent effect of rotating the focusing ring is greater than normal, so this kind of cancels itself out.
I've not seen it as an issue when I've used this combination. I need long telephoto lenses too seldom to motivate an EF 600 mm f/4L IS II USM or similar.