Quoting Google for simplicity "The observer effect is the fact that observing a situation or phenomenon necessarily changes it. Observer effects are especially prominent in physics where observation and uncertainty are fundamental aspects of modern quantum mechanics.".Not really sure what you're implying here. I've played at least a dozen different 5e campaigns at this point. Of them, exactly one has had the "acceleration" the person I replied to spoke of.
If it's supposed to be how the game is run, why is it so hard to find it in practice, across wildly different groups, from different contexts, where the one and only factor in common is that I happened to be a player in them?
And if the fruits of the so-called "DM Empowerment" initiative are to empower most DMs to make their campaigns suck more by not doing what the books plainly tell them to do, I'm really not sure it's as worthwhile as folks claim it to be.
The acceleration of leveling rates in 5e is simple math not impacted by the observer effect but you are very strongly implying otherwise to dismiss the math when you are the observer