I agree that it slows down considerably in the final third. I enjoyed it, but it was more there to convey how he isn't going to escape some consequences, and that he won't have the glory he wants. By this point we've already seen some personal consequences, as with Jean and his relationship with his wife. The communists on the team at Los Alamos also face consequences, which is contrasted to his tragedy (which is rather more pathetic), of people both a world icon and deprived of the social power to correspond to the technological power he has helped unleash. The state turning on him was a necessary consequence, and I guess Nolan wanted to do his classic interweaving of time frames and narrative by having a key betrayal come from the narrator. He is also just a disposable human being, a weapon, for the American military industrial complex. This implies even more strongly the Red Scare climate of distrust. I enjoyed it but I agree it was the weaker part of the film.

Nolan is still of course fixating on the tragedy of a 'human, all too human' man. Just because I can critique it as Marxist in whatever way that might be, doesn't really take away from any of the positives that such a story can still have. I've never really understand looking at art otherwise. People can still read the Iliad and come away moved and informed by its strengths as art without having to have a noncritical relationship to it or to the society it was created in and to the views of the social class whose ideals it represents. It seems to me reductive and immature when I see people claim that all art that isn't explicitly calling for revolution is evil/reactionary/shouldn't be liked, or whatever (I'm not saying that you think this). Art in the most basic sense is simply a social activity in which human beings produce meanings, ideas and narratives to give meaning to their lives. It can of course have other functions and properties, especially since it exists in a class society, and while one duty of art should be political, the latter in no conceivable way exhausts what art is. Sometime people want flowers and explosions because they are exhausted and alienated by their lives, and that's fine. The fact that people turn to art in their alienation is not at all necessarily an issue to me, no more than seeking psychiatric help.