TL;DR - This mod changes the recipe time for rocket parts from 3.00 seconds to 2.85 seconds, such that a fully-moduled rocket silo will produce 1000 (1001.67, due to rounding) SPM instead of 984 SPM. This is to assist megabase players who don't find the 984 SPM as "good enough" and do not want to build an entire second rocket to compensate for the missing science.

The actual description:

The rocket silo launch animation lasts 2420 game ticks, or approximately 40.33 seconds. To be able to complete one full construction/launch cycle within 60 seconds, the construction would need to take ~19.67 seconds - however, with the base recipe time, it will take ~20.6 seconds. As such, the most a single silo can produce is 984 SPM.
[ source: https://wiki.factorio.com/Rocket_silo#Maximum_throughput ]

For some, this might be acceptable, but for others (such as myself), being close isn't enough.
As such, the base recipe time for a rocket part has been changed from 3.00 seconds to 2.85 seconds.

The math behind it:

A rocket part at a crafting speed of 1 takes 2.85 seconds to craft with this mod enabled. With 20 beacons (each containing two speed 3 modules) and four productivity 3 modules within the silo, a rocket silo's crafting speed reaches 10.4, and will take 2.85 / 10.4 = ~0.274 seconds to produce each part.

A rocket needs 100 rocket parts to be built when there is no bonus productivity - with bonus productivity, however, that number goes down to (100 / 1.4) = ~71.429 rocket parts. Multiply the parts needed with the amount of time needed to craft each part, and we get 71.429 * 0.274 = ~19.572 seconds to build the rocket.

You may be curious: why not match it up evenly to 19.67 seconds, as we mentioned before? The answer is simple: rounding issues. Each recipe's time can only be narrowed down to the hundredths place, and as such I cannot work with the "true" value of what each pack should require. If I were to use 2.86 as our recipe time rather than 2.85, our time becomes too long, at ~19.92 seconds. Going lower (i.e. to 2.84) just raises the question of "why not make it 0.01?" and to that I give a big fat NO.

Anywho, this is my first mod - I hope you enjoy.
Shoutout to Goose for helping me rummage through the files and guiding me on my way through the making of this mod.