The positives of this movie will be extolled in a million other reviews -- incredible set/costume design, incredible camera work, and unbelievable acting. All three lead actors give award-deserving performances, especially Lily Gladstone (who has the least lines, but fills her screen-time with gravity.)
While I'm not opposed to a long run-time, I do think it must be justified by leaving not an ounce of fat, redundancy, or indulgence in the storytelling. This is where Killers of the Flower Moon falls short. The film's middle devotes itself so entirely to the minutia of the white murderers' plotting, it loses its sense of momentum AND, critically, leaves many of the Osage characters' storylines underbaked.
-- SPOILERS AHEAD --
The pacing of the first thirty minutes of the movie is fantastic. Ernest's introduction to Hale and Mollie, punctuated by visual showcases of the Osage culture, ranks among Scosese's best cinematic sequences. But as soon as Hale spells out his plot to Ernest -- to murder off the Burkhart sisters so that Ernest inherits their wealth -- the movie exchanges curiosity for dread. We know what's to come, and there's little hope that the worst will be avoided.
There are plenty of great movies that operate with an impending sense of doom. But dread does not pair well with tedium. I have not read the book the film is based on, but I can only imagine it explores every stray detail of Ernest and Hale's crimes in a way that doesn't translate to film.
An example -- Hale determines that Mollie's sister and brother-in-law must be killed via explosives. He and Ernest meet with a cowboy who knows an explosives man but doesn't know his exact location. We then drop this storyline, wandering to other treacheries, only to return to have TWO MORE EXTENDED SCENES of Hale giving Ernest instructions, then Ernest talking to a cowboy who knows the explosives man but is unwilling to track him down. While I'm sure this cowboy-phone-tag did occur in real life, must we watch the same scene over and over? Especially when we know without a doubt what will happen?
Also, remember when Mollie, deathly ill, traveled across the country to plead her case to the US government? You don't? That's because it only lasted two scenes and barely got as much screen-time as Robert DeNiro's driving goggles. I'll again compliment Gladstone on creating depth, poise, and humanity out of scenes that consist mostly of her in a sickbed. But a better movie would have treated this as a true two-hander -- the story of Ernest AND Mollie circling the dreadful truth at the center of their marriage. It's not the run-time that's the problem. It's the distribution of whose stories are told.
Three stars for the performances and some excellent film-making by one of the best directors to ever do it. But don't let the film-bros bully you. The modern audience is within their rights to expect storytelling discretion in a nearly 4 hour movie.