Forge
Minecraft with Forge mod loader
One Main Thread
The world tick runs on one main thread, so per-core speed matters more than core count. More RAM only helps until the server has what it needs. Past that point it can cause longer garbage collection pauses. Real gains come from lowering simulation-distance and view-distance, pre-generating the world (for example with Chunky), and profiling lag with spark. Heavy modpacks usually need 6 to 10 GB.
Pick the Right Java Version
Use Java 8 for MC 1.16 and older, Java 17 for 1.17 to 1.20.4, Java 21 for 1.20.5+. Mismatches usually surface as UnsupportedClassVersionError at startup.
Some MC Versions Have No Recommended Forge
For Minecraft versions without a "recommended" Forge build (for example 1.21.1), set Build Type to latest in the Startup tab.
Server Mods vs Client Mods
Players need the same Minecraft and Forge version, plus every mod that runs on both sides, in matching versions. Client-only mods (shaders, minimaps, Embeddium, Oculus and similar) must not be uploaded to the server. Remove them from your modpack's mods folder before uploading, or the server will crash on startup.
EULA and RCON
On first start the server stops and asks you to accept the Minecraft EULA. Read it, set eula=true in eula.txt from the file manager, then start the server again. RCON uses the password and port from your Startup tab. You don't need to touch server.properties.
Official Documentation