The bug

Skeletons or illusioners while riding on striders can move extremely fast, even if the strider is not in lava. When in lava however, they move insanely fast.

To reproduce

1. In the overworld:

   /summon minecraft:strider ~ ~ ~ {Passengers:[{id:illusioner,HandItems:[{id:bow,Count:1b}]}]} 

2. Go to survival
   → The strider starts moving very fast while the illusioner is attacking
3. Use the same command in the nether above a lava lake
4. Go to survival
   → The strider is even more faster in comparison to spawning one in the overworld (with no lava nearby)