Skill Haven adds a custom lot trait to Build Mode that doubles skill gain rates for every Sim on the lot. Every single skill in the game gets the 2x boost, so whether your Sim is painting, cooking, working out, or learning an instrument they'll level up in half the time. The bonus applies to all Sims on the lot, not just your active one.
To use it just go into Build Mode on any lot, open the Lot Traits panel next to the lot name, find Skill Haven in the list and add it, then head back into Live Mode. Remove it the same way whenever you want to go back to normal skill gain rates. No settings, no configuration, just drop the package file into your Mods folder and it's ready to go.
Requires base game only. No dependencies.