The bug

Using a Riptide trident while flying with Elytra can cause the player to fly at extremely high speeds, much faster than can be achieved using firework boosting. This only occurs if the player uses a Riptide trident while not currently under a firework's boosting effect.

It seems that Riptide tridents will keep accelerating an Elytra flyer without an upper speed limit, unlike firework boosting (using more fireworks while already being boosted will not cause a noticeable speed increase).

To reproduce

1. Set /weather rain
2. Give yourself and elytra and some fireworks with duration 3 (1 paper and 3 gunpowder in a crafting table)
3. Enchant a trident with any level of Riptide
4. Open the F3 coordinates display
5. Fly with the elytra while constantly using fireworks; notice that a single firework accelerates you to maximum speed and the speed doesn't keep increasing.
6. Fly and throw the trident several times; notice that the speed rapidly increases after each throw.

Demonstration video; notice how fast the Z coordinate is changing.