Walking on floating boats while they are attached to a lead isn't smooth.

What I expected to happen was:
The player would have walked easily on the boat without any problems, no glitches, nothing.

What actually happened was:
You can't move properly (frames drop too), it's mostly hard to sprint and therefore cannot be used in hard parkour courses. You are going all the time just a bit up and just a bit down because the boat is still not 100% stable and moves just a bit up and down when standing on it. This makes the movement of the player worse.

Steps to Reproduce:
1. Create a new world. (any)
2. Put a fence at least 7 blocks up from the ground.
3. Place a boat (either regular or a chest boat).

4. Attach the boat to the fence using a lead.

5. Get on top of the boat.

6. Start walking on the boat's hitbox.

A solution (for now):

When the boat is moving up and down less than 0.01, make it stop moving or remove its gravity.

[NOTE: This happens when the boat moves around too! By going away from the fence before the lead breaks and shift out, while you get on top of the boat it has the same problem. A non-attached boat works just fine.]

I have tried my best to also give you a video of the issue. It will be the best if you could fix it in 1.21.2 (Or just relaunch 1.21.1).