I'm sure that the Apple Engineers can make the screen light up at a reasonable time...
"Can"? Multiple reviewers of the watch which is in PRODUCTION mentioned the lag and called it "distracting", "frustrating", etc.
So, apparently, Apple Engineers couldn't.
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All these bugs will be easily fixed with future software updates
Not a chance. The watch is a black box with an accelerometer in it, which only can tell you the orientation and magnitude of apparent gravity relative to watch's sides, but cannot know if you're actually looking at the watch or doing something else. There's just no way around it. You either trigger the screen too much which will eat the battery for lunch and perform unwanted actions, or it'll lag annoyingly. There's no way for a dumb black box with just an accelerometer and no eye tracking camera to do the right thing 100% of the time.