When a non-player entity is riding another Mob. The Passenger's movement speed is used instead of the movement speed of the entity that is being ridden.

I already know that the mods know about this, however it was always brought up in an issue primarily about using the Passenger AI instead of the Ridden AI and so it was marked as resolved as the way the AI is controlled is working as intended. This issue pertains only to how movement speed is allocated rather than what AI controls the stacked entity.

This is observable in Skeleton Trap Horses, Spider Jockeys, Chicken Jockeys and any other combination of Mount a player summons into the game.

The best way to observe this is by summoning a stacked entity like so:

/summon zombie ~ ~ ~ {Passengers:[{id:"minecraft:zombie",Attributes:[{Name:generic.movement_speed,Base:0.05}]}],Attributes:[{Name:generic.movement_speed,Base:0.25}]}

The movement speeds of both Zombies have been changed so that the Zombie being ridden is much faster than its passenger. However, the slower movement speed applied by the Passenger Zombie is what's being used. By killing the Passenger, the ridden Zombie can now use its movement speed.