You are probably reading this article because you are experiencing performance issues on your Aternos server. There are different kinds of lags and an important first step is to identify what exactly is lagging. This article will help you with that.
Server lag / Low TPS
Minecraft calculates game updates (e.g. plants growing, redstone changes or mob movements) at regular intervals. All of these updates together are called a tick. Minecraft tries to run a new tick every 50 milliseconds, which results in 20 ticks per second. Those Ticks Per Second are often shortened to TPS and are an important metric to measure Minecraft performance.
If Minecraft can't calculate a tick in 50 milliseconds, then the next tick has to wait for the previous tick to finish. Therefore the game can no longer calculate 20 ticks every second and will slow down which you might notice as lag. When playing online, ticks are entirely calculated on the server, so lower TPS are an indicator of server performance issues.
On Aternos, you can see your TPS on the server page while the server is online. This only works with Minecraft: Java Edition on versions 1.16 or higher.
If the TPS are lower than 20, then your server is lagging. You can read our article on server lag for potential causes and solutions to those issues: Server lags
If your TPS stay at 20, your server is running fine and you are experiencing a different kind of lag.
Client lag / Low FPS
Minecraft on your computer is called a client in contrast to the server, that runs on Aternos. The client connects to the server and displays the game for you. To do that, it generates a new image of the current game every few milliseconds and displays it on your screen. This image is called a frame. How many Frames Per Second are generated is measured in a metric shortened as FPS.
On Minecraft: Java Edition you can see your current FPS by pressing the F3 key on your keyboard. The FPS are displayed in the second row at the top left:
60 FPS means that 60 new images/frames are generated every second. Many screens can only display up to 60 frames per second, so this is a good and normal value. Lower values can also sometimes be normal, but everything below 30 will be quite a noticeable lagging experience for you. The game will visibly stutter and your movements feel delayed.
You can read this Minecraft wiki tutorial on client lag for potential causes and solutions to those issues: https://minecraft.fandom.com/wiki/Tutorials/Improving_frame_rate
Network lag / High ping
Your server on Aternos is hosted on our machines in different data centers in Europe. If the connection between your computer through your Internet Service Provider (ISP) to our machines hosting your server is unstable or too long, you might experience network lag. One indicator of the quality of your network connection is the connection icon in the player list when pressing the TAB key in Minecraft: Java Edition
It's important to note that your internet speed/bandwidth is not the only relevant factor for the quality of your network connection. You can imagine the internet like a big road network with smaller and bigger roads and while your connection to a local website might be good (and speed tests are usually to local servers), the route to a destination far away might still be congested and delayed.
Similar to roads, internet routes are sometimes under higher load and the issue will just fix itself when the load reduces. There isn't much you or we can do about that and waiting is usually the best fix to short-term network issues. If your issues stay for a longer time, you can read this article to learn how to collect diagnostic information that you can send to us to maybe identify the issue: Collect diagnostic information about your server connection