When your server is lagging, or your players are complaining about lag you will need to figure out what is causing it so you know the root of the issue.

Lag falls under two main categories here, client side or server-side. Server side means it is the physical server that is the problem, and shall be causing lag for all the players online. Client side however is out of your control and it is your player, or players who see the lag.

CPU & RAM
This is server-side lag, if there is an issue on the server’s side causing lag to your players you should check your servers console for errors. If you see a warning like the one below:

CONSOLE: thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running [####]ms behind, skipping [##] tick(s)

This warning means that your server is not running at Minecraft's needed 20TPS, usually if there is heavy lag your server will be running below 15TPS. This may be a sign that it could be either your CPU or RAM that is slowing the server down.

You may check how much RAM your server is using by opening the client to see how much it is using out of the RAM you allocated it. If your RAM use is above 90% then this could likely be the issue and you will need to give it more.

If however, the RAM use is fine it could be your CPU is not fast enough. If you are hosting your server on your own computer, it may be the time to buy professional hosting.

Ping
If there is nothing wrong with your server but users are still lagging this could be an issue with the connection they have to your server. This results in a high ping meaning there is a big delay between packets being sent and received.

You will see on a server the strength of people’s connections in the Tab menu when you are connected. You will see this strength in bars.

If you are hosting your server yourself on a home internet connection it could be that the speed is not fast enough. It could also be that users own connections are not fast enough to connect.

If you have bought hosting you may want to change which country your server is hosted in, you will ideally want it as close to your player base as possible.