>Are they moving faster than conceivably possible by a real player? Even the most basic (x2-x1)/t > twice the theoretical will catch people teleporting or speed hacking.

This is how I imagine Amazon ended up banning a large amount of players for speedhacking. The players were lagging. I'm guessing their anti-lag features ended up moving them faster than the anti-cheat expected.

But I agree that a combination approach would probably work.