A game server is a simulation loop with a fixed deadline, and no amount of cores makes one thread finish sooner. What to buy for a single instance, what to buy for density, and why jitter beats port size.
A stock kernel moves a few gigabits and then flattens. The bandwidth-delay arithmetic behind it, the sysctl ceilings to raise, the NIC queue work that follows, and the nginx settings that stop copying bytes.
The first fifteen minutes after handover: a real user account, key-only SSH, a firewall that defaults to closed, and updates that install themselves.