Hmm... I don't have the problem the dpreview describe unless in low light condition.
I did a test quite some time ago using 6D at a target around 50m and the initial AF speed of the Tarmon @400mm in One-Shot AF is as fast as my Canon EF 400mm F5.6L. Didn't test in AI Servo (not sure how to measure the time taken), but I feel the Tarmon is a bit slower than my Canon 400mm F5.6L in AI Servo.
May be should re-test using 7D2.
Have a nice day.