Encounters are "dumbed down" when they're reduced to a fixed script, proceeding along a fixed timeline, with perfunctory variations between runs.

Having mechanics thrown at you more quickly is certainly a form of difficulty, but that doesn't substantively change the intellectual challenge provided by an encounter. It just means you have to crank up the speed of your internal auto-pilot, or have twitchier reflexes.

Not-dumb encounters would entail there being significant amounts of on-the-fly decision making.