The bug

Ridden mobs use the path of the riding entity with a fixed and rather high speed which might be twice as high as its own speed. The expected behavior might be that the mob uses its speed instead.

This can be seen for example with spider jockeys or skeleton traps.

How to reproduce

1. Summon for example a skeleton riding a spider

   /summon spider ~ ~ ~ {Passengers:[{id:"skeleton"}]}
   

2. Switch to Survival mode
   → You will see that the spider chases you faster than normal

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.entity.EntityLiving.updateEntityActionState() sets the path of the ridden entity to the path of the riding entity with a fixed speed of 1.5.