I really like the 135mm f/2.0L, however, I don't use it as much as I used to because it is starting to show its age. The lens isn't much faster focusing than the 85mm f/1.2L II (feels like it has slowed down over a few years) and I think the focusing speed needs to be improved to keep up with the 1DX. 135mm is one my favorite focal lengths for basketball so I'm looking forward to a new model. I don't need the extra 1/3 stop of light or IS but will happily take any improvements I can get my hands on.Interestingly, this LensRental article published today might just agree with exactly what you wrote
The two newest Canon cameras have more accurate phase-detection sensors than their previous cameras. The newest lenses have more accurate focus movement (or provide more accurate focus movement feedback, or both) that takes advantage of those sensors. Older cameras don’t have accurate enough AF sensors to take advantage of the new lenses’ capabilities. Older lenses can’t move their focusing elements with enough accuracy to take advantage of the new cameras’ accurate sensors.So, a lens might appear to slow down if you've upgraded bodies over time, or, if you've gotten a newer lens that pairs better with a newer body. The 135L being a 1996 lens would suggest that's likely, and so an upgrade to it might make an already fast lens even faster.