Today I found out that covering areas with slabs to prevent mob spawning causes lag. On a large scale, it can cause so much lag that it uses 50% of the CPU of the whole server. So if you use slabs on a few places to prevent mob spawning, it’s fine. Once you count the places where you use it in the hundreds, or if you cover whole chunks with slabs, you are creating an issue.
The problem comes from the fact that the mobs spawn and de-spawn again on the slabs, as opposed to properly lit areas, where the spawning is prevented before it happens. When the mobs de-spawn, the server has a lineup of mobs to populate the chunk, and it starts from scratch.
So please do not use the slabs as a broad method to prevent mob spawning.