Tracking and focus with the 7D?

Had another track meet today. I trouble with people getting being in the way when trying to shoot. Especially during long jump and triple jump. I have tracking sensitivity to the slowest. My question is how long should it take to get the subject back in focus?

I had some shots that were out of focus because I was not able to get the subject back in focus fast enough.

I am comparing photos and times, and one set looks like one second from focus shot to focus shot. But I have a few in between that are out of focus.

I assume this is just something I have to deal with, unless I can shoot in a spot where nobody is standing, walking in your way etc. Uh not when I am shooting.

Is that a murphy's law? DH said just when you needed to record someone would walk in the way of the camera. Luckily I got SOME good shots. Sadly If it wasnt user error it was someone in the way.