Project ZomboidGuideSkill leveling speed is one slider on your dashboard: XP Multiplier. Set it anywhere from 0.1 to 20, hit Save, and it applies on the next restart. Nobody loses anything: every point of XP your players already earned stays exactly where it is.
Open your server's settings and find XP Multiplier.
Pick a value between 0.1 and 20. 1 is the normal game rate, 2 doubles all XP gains, 0.5 halves them.
Save, then restart the server.
The multiplier applies to all skills, and it changes the rate going forward. It does not recalculate past XP in either direction, so you can experiment freely.
Want 3x Cooking but normal everything else? The dashboard slider is one global rate, but per-skill rates are available through the Sandbox Overrides box at the bottom of your settings, one line per skill:
MultiplierConfig.Cooking = 3.0Overrides win over the settings above them. The full override syntax is covered in "Project Zomboid: Change Any Sandbox Setting".
Making a new character. The XP rate is server-wide, not per character. A fresh character levels at the same rate as everyone else.
Expecting existing skills to jump. The multiplier speeds up XP from now on; it does not retroactively re-award anything.
Restarting without saving. Settings apply on restart, but only after you hit Save.
Want more build points at character creation instead? The Free Trait Points setting (-100 to 100) adds points during character creation. It is read once when a character is created, so it affects new characters, not existing ones.
Every sandbox setting from the dashboard: see "Project Zomboid: Change Any Sandbox Setting".