Get Elytra Flying Speed?

I am making a mod where if you have a certain effect, and you are flying above terminal velocity with an elytra, when you make contact with a surface, you explode.

I don't know how to get the player's speed while they are flying, and I also don't know what an elytra's terminal velocity, (the fastest an object can go until it stops accelerating), even is!

Is it even possible to get the player's speed while they are flying with an elytra? If so, how would I do it?

And what even is the elytra's terminal velocity, as in the fastest it can go?

Any help would be greatly appreciated!