Natural Selection 2
Natural Selection 2 Dedicated Server
Steam Account Required (Not Anonymous)
NS2's dedicated server cannot install with the anonymous Steam user. You must supply a real Steam account in the Startup tab. The account should own NS2 for clients to connect reliably.
Single-Threaded Spark Engine
NS2 runs on the Spark engine and is single-threaded for gameplay logic. Adding cores won't help. CPU clock speed and entity count do.
Official Documentation