This article covers several ways to find and solve performance issues. Minecraft is a complex game and lots of factors influence the performance of a server. Make sure to carefully read this article and follow the instructions to improve your server performance. Please note, that it's not possible to guarantee a lag-free experience especially for a free service.
Use Paper
PaperMC is a software that contains several enhancements over the vanilla version to improve performance. Additionally, it supports the installation of plugins.
You can install it here: https://aternos.org/software/papermc/
Admittedly, PaperMC does not solve all problems and the performance is still not outstanding but still worth the switch. Make sure to update to the newest versions, they always contain the latest improvements and corrections.
Gameplay
There are a few other tricks on how you can reduce lags.
Mobs: For example, mobs have a big impact on performance. We do not recommend keeping many mobs (animals or monsters). Limit yourself to a few.
Items: Having many items on the ground can cause performance problems. This can for example happen when you give a player a very large number of items through the /give command. You can delete all dropped items on the ground with the command
/minecraft:kill @e[type=item]
Random tick speed: The random tick speed is a gamerule that changes how often certain random events in a world occur (read more here). Setting this gamerule to a high number can harm the performance. You can set it back to the default value of 3 using the command
/minecraft:gamerule randomTickSpeed 3
Also, loading chunks has a big impact on performance.
Exploring: Exploring new areas causes large lag spikes and decreases performance massively
Loaded chunks: It is also worth reducing the number of chunks loaded, especially if you play with several players on one server. This means that if there are many players close to each other, it is better for performance than if there are many players far away from each other. We already reduced the number of loaded chunks for 1.13 and newer servers, you can find more information about that here.
Do you use plugins or mods?
Too many or misused mods, plugins or worlds can cause server lags. Make sure to install a reasonable amount of plugins and don't create too many worlds. Mod features, e.g. machines or chunk loaders also can cause lags. To identify the problem here, it could be helpful to check your log (https://aternos.org/log).
It is also possible that a single mod contains a bug or is just not really performant and causes on its own to slow down your server. Try to remove mod by mod to find out if you are affected by such.
Do you use a modpack?
We always try to get the best possible balance between our limited resources (since we are free, and we don't want to change that) and the best performance. Especially for modpacks, this is really hard to achieve. We test the modpacks in our system to ensure that they are at least starting and that basic playing is possible. Because modpacks are always a bit different and the load on our host systems is also always different, we cannot guarantee anything here. But we decided to still offer modpacks on our platform because we want to offer as many options to customize your server as possible, and most of the modpacks are pretty playable most of the time.
Is your internet stable?
If your internet connection is bad or temporarily overloaded, lags may occur. To test this, you could try playing on other servers and check if there are any lags.
Other reasons
It is not really likely, but possible, that an Aternos host machine is temporarily overloaded. This should be only for a few minutes and only occur rarely. In this case, you can only wait or completely stop and start (not restart) your server.
Do you need more performance?
Aternos is a free service and therefore limited in the resources that can be used to run the servers. If you need more RAM with better performance, and you are able and willing to pay, you can try out our paid service exaroton: https://exaroton.com.