Minecraft server is lagging, why and what can I do?

I bought or rented a server with a friend on serverminer.com.

The server has 6GB of RAM of which 15% is used, the CPU at the server never goes higher than 33%

Now the following problem.

From now on the server started to lag. The chunks are not loading properly and are only super slow. If you drop things it takes min. 10sec. Until they disappear from the inventory, the same thing happens when collecting items. Even messages or commands only written in chat are delayed.

We're building a large map have worldedit, building gadges and optifine as mod, so it can't be because of that.

Minecraft version 1.12.2, the support of the serverminer side is unfortunately a long time coming, so I hope for a solution here.

Here is a picture of the world. Thanks in advance

First of all, 33% is already a lot… A maximum of 20% should come, as this is only indicated by the server load and not by the total load. Have you already looked for possible chunk errors in the console? Or do the versions not match? Or maybe it is also because of the server provider that they just have too bad servers and too many are using them at the moment.

The versions are correct. About the chunk errors We haven't done anything yet, how does that work exactly?

That should be in the log

How can I look for errors in the console?