Film is expensive.

They are not talking faster, you are just listening slower.

My second answer. actually, it may sound this way, because of the variance in the equipment used. the older cameras could have been recording at a little slower speed, than todays modern cameras and playback equipment. This difference would make a difference in their voices.

I don't think it's that they talked faster rather that they spoke more clearly than modern actors. They had crisp and clear enunciation that today's mumbling method actors just don't have. FIlms made in the 30's and 40's relied a lot more on dialogue than films made today and as a result actors had to be able to act unsupported by special effects diversions. It was a different sort of acting and more like stage acting. Modern actors know they can rely on hypersensitive sound recording equipment to capture even the faintest mumble so they don't learn how to project their voices. Because they don't form their words clearly they sound slow. Compare Russel Crowe's indistinct mumblings in Gladiator to the delivery of studio actors in similar films from the early days of Hollywood. Watch Spartacus and see. As Crowe delivers his lines to Commodus in the Colosseum in almost a whisper we are to believe the entire audience hears his words. Come on! Anyone watching films from the studio era of Hollywood will be blown away by the clarity and depth of the dialogue. In the modern era SFX have killed acting dead. Today's mumbling megastars wouldn't have got through the door in the 30's.

because they got the new fast talk app on their iPhone?