It literally takes not even a half second for the display to light up and tell me the time when I rotate my wrist. If it was a full second or more, your complaint may be valid. As for the watch detecting rotations, it rarely lights up when I don't want it to do so. Seeing as I made it the whole day yesterday with about half the battery left, it's clearly not affecting battery life, so it's a non-issue for me, at least.
For my original Pebble, (although it always displays the time) when I want it to light up, it requires some pretty serious wrist flicking to do so. Not very consistent at all. Also, I've tried on the Moto 360, and it's pretty hit-or-miss as well. Apple Watch is the most fluid out of all the smartwatches I've tried so far.
There are a good number of valid complaints with the Apple Watch, but I don't think this is one of them.