What determines a movies pace 100% of the time is the genre.
-Rom Coms (Romantic Comedies) are pretty fast since they are usually 90 minutes to 1 hour and 45 minutes (105 mins).
-Action Movies (and subgenres) are usually 120 minutes (2 hours) and possbily more (I.E. LOTR Trilogy; Star Wars films; Matrix series).
-Drama (and subgenres including Romantic Dramas) are like action movies: 2 hours (or less) to three depending on what it is, I.E. Doctor Zhivago.
-Comedies (separate from Rom Coms) are usually 85-95 min. maximum for the simple fact you can't maintain the core joke for over two hours a good 99% of the time.
This has everything to do with the pacing of a film because studios give the director specific time constraints given the genre he/she is working in because a shorter movie -- even a 90 min. action movie -- Means more showings at the local multiplex which = more revenue.
This is why, if you haven't guessed already, epic length movies like "Troy" and "Titanic" aren't made that often... And if they are... They are spread out among genres as to not saturate the market with long movies that require longer showing times at the theater.
Also, to the person that said they don't like movies where nothing happens...
There is always SOMETHING happening in a mainstream film versus an independent, avante-guard film that has, for example, one shot of a tree for 15 minutes and then something else happens and the film is over
Multi-million dollar films don't get made in Hollywood unless "something" is happening.
You may not think that "something" is very interesting... But trust me. There is ALWAYS something happening even if it is so subtle that you don't think there is.