Early sound films were concentrated on just that - sound - with the visuals often suffering as a result. Keaton was able to insert little bits of silent business in them, but when compared to his silent work, they frequently seem slower paced. This is because - they were. Sound film is filmed and projected at 24 frames per second, based on the projection speeds in place in the late 1920s, speeds set to increase the number of daily projections of the features of the time.
Silent film began at being shot at 16 frames per second (to squeeze in the most action within a limited film budget, Griffith had his Battle at Edlerbrush Gulch shot at 14fps!). So, filmmakers caught onto the increased projection rate by increasing their camera speed to about 20fps by 1920, and more like 22fps by the mid to late 1920s. Actors, editors, and directors became used to that rhythm and performed accordingly.
When 24fps became the standard from 1928 onward, it took a while for the pacing to accommodate the change, and especially visual humor. Keaton's early MGM work is a case study in this. First of all, MGM didn't understand humor, shooting and lighting their comedies in the same manner as a courtroom drama (just look at the dreary sets of Speak Easily).
For a visual comparison, here is a silent sequence (with ambient sound, no dialogue) from the frustratingly staged Parlor, Bedroom and Bath from 1931. This is how it looks, shot at 24fps, the standard sound camera speed. The gag is a variation from the house-demolition climax of his 1920 One Week:
Now, if it had been shot at 22fps (about 10% "quicker" during projection), the standard for the mid-1920s, it would have appeared as this:
And if it had been at 20fps (20% quicker during projection), the general speed of around 1920, it would have appeared this way:
The differences are subtle and work on a subconscious level - the eye can absorb the information faster, so what we are left with is the pacing. At 20-22fps you can see how the performers of the silent era were thinking things would appear during the early sound era.
When The Artist was produced in 2011, it used the 22fps camera speed - and it worked very well on many levels that the Brooks film, Silent Movie, failed to achieve in 1976.
Keaton's work at MGM showed many production blunders besides misunderstanding visual humor - my film production students were able to shave 10 minutes off of Speak Easily by following basic editing techniques - avoiding redundancy, punching a joke by proper cutting, creating inserts to direct the audience attention - and were working on Doughboys to experiment with varying film speeds during the silent sections. "We can fix it in post," may have been a phrase used then, but rarely employed.
(The use of varying camera speeds is still employed today and not only for comedic effect - Raiders of the Lost Ark used 20-22fps for some of the action sequences. When blended with music, sound effects, and editing, nobody really notices that things are moving 10-20% faster than might be humanly possible. Movie magic!)
Timing can work both ways. Here is the original gag from Keaton's One Week in 1920, projected at 24fps, followed by a projection at 20fps (both clips are silent):
24fps projection
20fps projection
And, regarding the 1920 film, it is interesting (to me, anyway) even how carefully the For Sale sign is positioned, how it stays rigidly in place with its gentle posing, and how the instructions envelope sits at the right spot - nothing wobbles, nothing moves, with all that surrounding destruction.
