"The server is laggy" describes two completely different problems with two completely different fixes. Working out which one you have is the first step.
Server-side lag
The server cannot finish its work inside the tick budget. Everybody experiences it equally, regardless of where they are connecting from. Look for tick warnings in the console and profile the server.
Network latency
The server is healthy but the path to it is long or unstable. Only some players are affected, usually the ones furthest away. Server tick rate looks fine.
Telling them apart
- Check the server's tick rate in the panel or in game. If it is low, it is a server problem, not a network one.
- Ask whether nearby and distant players are equally affected.
- Have an affected player run a trace to the server address and look for jitter, not just the average.
- Check whether the problem correlates with player count or a specific time of day.
Inconsistent latency is more noticeable than high latency. A stable 90 ms plays better than a 60 ms average that spikes.