Regardless of your movement speed attribute, your horizontal speed while falling and the speed at which you can move underwater will not change. As a result of this, when speed is slower than normal, jumping while moving makes you go relatively faster, but if it is faster than normal, jumping will make you go much slower.

Note: This is very noticeable when used together with scale attribute, i.e. if you make yourself a 16th of your size and speed and jump height and then move forward while jumping.

To reproduce

1. Run the following command to decrease your movement speed:

   /attribute @s minecraft:generic.movement_speed base set 0.0001

2. Jump while walking
3. Swim in the water

To reset to regular movement speed, use the command

/attribute @s minecraft:generic.movement_speed base set 0.10000000149011612

Expected behaviour

The horizontal movement speed in the air and in water would also be affected by the movement speed attribute

Observed behaviour

The horizontal movement speed while in air or in water is the same as normal