1. Grab an Elytra and Fireworks
2. Either TP to a high height, or fly there
3. Face directly downwards and let yourself fall
4. Open the F3 menu for being able to compare values
5. Once you've sped up, start using fireworks
6. You will notice that your y-coords change slighty slower

I presume this occurs due to fireworks attempting to ascend the elytra, which works against the acceleration as you face downwards. Expected behaviour for me, would be to speed up the individual downwards anyways.

Video: https://streamable.com/3prh23