I have no qualms with a long movie. If the story is great I want to live in that world for as long as possible. A 95 min movie can feel like an eternity (good or bad) and a 3 hour movie can fly by and leave you wanting more.

The problem, as I see it, is unfocused storytelling.

It starts with a good screenplay that is focused, concise, and well paced. Shoot more than you need to tell the story, then edit out everything that's not 100% necessary. Be lean and purposeful in your story. A great screenplay will service everything in the story with great economy. Great scenes will serve two or more purposes, develop the story, and - when they're really great - make you lose all sense of time.

Many films are full of sprawling superfluous plots that don't amount to much. I feel this most during certain blockbuster films that are packed to the gills with spectacular set pieces, have three separate "emotional" climaxes, and somehow fail to stick the landing. When these elements are poorly balanced or overcooked they feel like a slog.

I would not phrase the means of approaching best length as "edit out everything that's not 100% necessary" but as something more like "make every scene count/pull more than its weight". The former, to my mind, tends to associate with small goals and a pure utilitarianism that falls short of excellence. I doubt you meant anything like that.

I certainly agree that "Great scenes will server two or more purposes". A scene can advance the plot, display who the characters are and who they may become, foreshadow or rationalize future plot elements (or present red herrings), introduce worldbuilding depth, manage the emotional state of the audience (comic relief is a simple example), etc. I would not be surprised if a great scene can also so elevate other scenes that its relative greatness may be less obvious because it makes the whole greater than the sum of its parts in isolation.

I very rarely watch movies (even less often in a theater) and I do not have a drama or fiction writing background, but even a "little learning" about the theory hints that bulk is easier to achieve than extensive excellence.