The bug
Mobs cannot pathfind properly and thus move very quickly on blocks without a solid top face, especially if there is air or another block without a solid top face below.
This is known to affect the following blocks (list may be incomplete):
Bottom slabs
Farmland
Dirt paths
(Empty) end portal frames
Enchanting tables
Carpets above string
Chests
Campfires
It seems to be affecting all mobs, though it seems like the issue is best reproduced with skeletons due to their strafing.
How to reproduce
Create a "Redstone Ready" flatland world
Run (optional, but makes the issue easier to see)
/fill ~ ~-2 ~ ~20 ~-12 ~20 airRun
/fill ~ ~-1 ~ ~20 ~-1 ~20 oak_slab(Or any other affected block)
Give yourself some skeleton spawn eggs
Switch into survival mode
Spawn a skeleton on top of the slabs
→ ❌ The skeleton moves really quickly
Example
Here is an example in video: https://youtu.be/rNsKKNCFxMM
Linked issues
is duplicated by 16
MC-159890
Skeleton Moving faster on tilled land
Resolved
MC-161232
Skeletons goes incredibly fast on slabs
Resolved
MC-171915
skeletons are broken on grass path
Resolved
MC-171955
Skeleton moves '1000'Mph on half slabs
Resolved
MC-174789
Skeletons on slabs sometimes can't hurt the player when they're close enough to them
Resolved
relates to 2
Attachments
Comments 15
Bedrock edition also has this bug MCPE-47075
version: 1.21 ...
mob still freeze when in slabs with a space of 2 blocks between 2 half blocks, slab on bottom and top
Can confirm. A similar effect occurs with waterlogged top slabs, see MC-156022.