Modpacks lag for different reasons than plugin servers. The tick is filled with mod logic: machines processing, pipes moving items, tech mods scanning inventories, mob mods spawning things.
First: enough RAM
A pack with 150+ mods wants 6-8 GB, big kitchen-sink packs 10 GB. Below that the server garbage-collects constantly and every symptom looks like lag. The modpack preview in our installer shows the usual requirement. Upgrade before tuning anything.
Second: server-side performance mods
Add these through the Mods tab (they are server-only, players need nothing):
- Fabric: Lithium, FerriteCore, Krypton, Spark, ServerCore.
- Forge / NeoForge: FerriteCore, Canary (Forge), Spark, ModernFix, Entity Culling is client-only (skip on server).
Check they exist for your exact Minecraft version. The installer only shows matching ones.
Third: find the hungry mod
Take a Spark profile. Modded profiles show the mod's package (mekanism, create, ae2) in the tree. Then:
- Create: too many contraptions or a giant train network; reduce or use the mod's
contraptionTickRatesettings. - Tech mods with pipes (Mekanism, Thermal): pipe networks scanning every tick; fewer, longer pipes; use conduits with buffer.
- Mob mods (Alex's Mobs, Ice and Fire): lower their spawn weights in the mod config.
- Chunk loaders from FTB Chunks etc.: cap per player.
Fourth: the world
Pre-generate with Chunky (works on Fabric/Forge too). Modded terrain generation (Terralith, Tectonic, BOP) is 3-5× more expensive than vanilla; exploring is what kills TPS in most packs.