Godfather 1 is one such movie. Try watching it with someone who hasn't seen it before before yelling at me, please - Its the slowest movie ever (now!).
Obviously you've forgotten about Once Upon a Time in the West. A fly gets more screen time than the bulk of the actors in the film. Almost no one from the lengthy opening scene is seen again in the film. Today the first fifteen minutes of the film would be reduced to less than 30 seconds of quick cuts. I can't say it's the slowest movie ever since I've yet to make it through Cleopatra or the soviet version of War and Peace.
Hah, thats true - I *loved* that movie as a kid, but can't bear to watch it anymore. Another favorite was The Maltese Falcon, and that still semi-holds up, but I can't get other people to watch more than 20 minutes of it with me. I guess my original memories of it are strong enough to overcome the older style of storytelling.