Paper
Minecraft with Paper
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), limiting entities and redstone farms, and profiling lag with spark.
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 and the in-panel console connects to it. You don't need to touch server.properties.
Official Documentation