You can't just go by ISO settings in the camera. The lens matters a whole lot when focusing in low light. The 300mm f/2.8L lens will focus much faster on moving objects in low light than the 300 f/4L. This has to do with which types of auto focus sensors the lens can access. If I were shooting evening football for instance, I would not use my 300 f/4L because I'd probably miss badly. The 300 f/2.8L never misses. It's both camera and lens that matters.
And I have a long, long list of file names of photos where I missed badly to prove it