What config to change

Change default simulation to 8 chunks.

Why should it be changed

This Reddit post and the link to the wiki within explains it better, but simply put, the mob spawning mechanics work like this:

However, if you lower the simulation distance from 8 chunks (and thus, lower from 128 blocks), this will leave mobs unloaded which makes them unable to despawn, but still contributing to the mob cap. This could mess up mob farms, and unless someone does their research they wouldn't know how to fix it, so I believe it's best to set the default to 8 chunks.

Additional details

No response

unable to despawn, but still contributing to the mob cap

What is your reference for that claim? The whole reddit thread just seems to be speculation...

I am now convinced based on the following:

• https://minecraft.wiki/w/Mob_spawning#Java_Edition_2 says

Mobs other than fish despawn immediately if no player is within 128 blocks of it, while fish despawn if no player is within 64 blocks.

• https://minecraft.wiki/w/Simulation_distance#Java_Edition describes that simulation distance affects ticking and visibility in Java Edition - not (de)spawning, which proves the previous point of 8 being useful for despawning cases
• https://minecraft.wiki/w/Simulation_distance#Range_of_simulation_distance says that 8 simulation distance is default for "low performance devices" aka 1 GB RAM or 32-bit on Java Edition - so Mojang prefers 8 over 6

To be tested in an upcoming beta, unless there are good counter-arguments.