This film was not boring or slow at all

You and your dumb fuck generation are the ones who bash movies for being over 2 hours.

I would think that an old-school filmmaker like Marty is always looking to entertain with his work, even if he might also like to do other things as well.

And like I said, I think it COULD'VE BEEN a more entertaining movie, while still sticking to the facts of the story, but he would've needed to go with his original impulse to focus on the FBI investigation.

Well I am a bit of an old fashioned kind of guy, so perhaps there's something to your comment about it being an "old fashioned" view.

However, you could perhaps switch-out "hero" for "protagonist" in some cases, such as Goodfellas. I'll agree that it has no "heroes," but the audience undoubtedly sympathizes with Henry Hill, gets caught up in his journey, and would like to see him ultimately succeed in his world of mobsters and mobster happenings. So you still have a central character who is likable and appeals to the audience, a character to root for and to hope succeeds.

In Flower Moon, you can't say that the Natives--or more specifically the character of Mollie--are the protagonists, because they do not occupy that space in the story. They are the victims and have no hope of succeeding by their own hand. And while Leo's Ernest is sometimes vaguely sympathetic, certainly we know that we can't root for him. And so the characters that ultimately fill the protagonist space in the story are the FBI agents, but they are mostly-nameless side characters who get little screen time and who are only directly focused on for about 35 or so minutes out of a 2 hr 35 minute movie.

As I mentioned earlier, Scorsese's original idea was to cast Leo as an FBI agent and have him be the story's protagonist, and I think that if he had done that a better film would've resulted.