It is a classic movie. That means everybody recommends it and nobody watches it. Hehe, ... sorry.
But it is true, the movie is hard to get into. It is slow, but only if you judge it by modern Marvel-like movies. It is more about creating a portrait of the man- Mr.Caul rather than some great event. I was about to drop it halfway, but then I read Coppola loved this film and pursued on. The ending was Brilliant!
So please be patient and you'll remember this movie fondly for many years to come.