To start, I'm going to assume you waste no time free hanging on the bar between reps, and that you are doing perfect form...
I'd focus more on the rep count, and keep the time the same, or ignore the time altogether . You only want to change one variable at a time
- time to do the reps
- amount of reps(speed)
The time to do the reps, if too long, will increase your endurance solely. The amount of reps within that time will increase your speed. Hypertrophy applies by the same rules with timed sets.. time under tension makes muscle growth, or around 8-12 reps will be optimal for hypertrophy. Look at this example,
Person a does 10 pullups in 30 seconds
Person b does 10 pullups in 60 seconds.
Assuming no one is resting during a free hang, person b has to do much slower reps, about 3 seconds up and 3 seconds down.. this is a LOT harder to to than person A doing 3 second pull ups.. as you can see, speed here does not equal greater growth.. it actually equals less. Slow negatives and concentrics really help muscle growth
Speed is its own factor.. look at this equation-
Power=Force x distance/time. Force=mass x acceleration.
To make the math simple, just reduce time in the power equation, which represents speed, or increase acceleration in the force equation. With less time, the power increases.. you can see you produce more power, but does not have anything to do with force. You will build speed, which will be great, especially if you plan to powerlift, as speed assists strength in producing more power.
In scenario 2:
Person a does 20 reps in 60 seconds
Person b does 10 reps in 30 seconds
In this case, you are doing more reps which will result in better growth but will mostly improve endurance. The time here does not matter.
To summarize, work on perfect form, and try adding weight to your pullups to assist in hypertroph, or do more reps but forget timed sets unless you want to focus on speed.