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.
- Summon for example a skeleton riding a spider
- Switch to Survival mode
→ You will see that the spider chases you faster than normal
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.