Mobs riding entities use the speed of the entity they are riding on instead of their own speed

The bug

Unlike in previous versions, mobs riding on entities use the speed of the entity they are riding on instead of their own speed. This causes certain mobs such as chicken jockeys to move extremely slow.

How to reproduce

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          Start by typing the following command in chat:

1. /summon chicken ~ ~ ~ {Passengers:[{id:zombie,IsBaby:1}]}

       2.  Observe the mob's speed