There's a passage in Stephen Levy's book "Hackers" where he talks about a robot built by some students at MIT that could catch a ping pong ball thrown to it by a human. I don't recall what year this project was done, but I do recall what computer they used: a PDP-6.

So a computer built 50 years ago is already fast enough to pass your instant-reaction test. (Well, admittedly, maybe it didn't respond instantly, but if it could catch a thrown ping pong ball, it was probably pretty quick).

This comment isn't meant to scoff at your suggestion -- far from it -- I think an instant-reaction-bot will still impress most people (and rightfully so). I just wonder if we, as programmers and scientists and engineers, haven't been taking as much advantage of Moore' law as we ought to.