When NetCode package present the Unity.NetCode.ServerSimulationSystemGroup produces warnings below if frame is rendered under 2ms. Making scene more complex or enabling vsync/gsync fixes the issue. Is it expected?
For now it is expected, we have a bug filed for it so we will investigate it further in the future. The problem is that the server is running at a fixed timestep, so when you render a frame in 2ms the interval between two server ticks is more than 4 frames.
When you make a server build we by default use Application.targetFramerate to prevent the warnings.