Memory
Too little Java memory is the most common cause of lag and crashes. See RAM.
Players
The game warns that more than 32 players can cause poor map streaming and desync. Max Players in Server Settings sets the limit.
Settings that help
- Zombies Count Before Delete: leave it at 300, as the game recommends.
- Car Engine Attraction Modifier: lower values can help with lag.
- Hours For World Item Removal and Hours For Corpse Removal: clear dropped items and bodies.
- Ping Limit: kick players whose ping is too high.
CPU priority on Windows
CPU Affinity sets the server's process priority and which CPU cores it uses, so it doesn't compete with your game on the same PC.
Questions
Should I restart the server daily?
Many owners do. A scheduled restart with a warning keeps it fresh and updates mods.