Server issues, Please help

[Yolopukki]

Dear Minecraft GURUs!

I am running my server (Tekkit Remastered, 1.12.2 from Forge) and it lags, but does not uses the resources available.
What should I do to make it run better?

[3XJ]

We do not offer support for other servers. I recommend looking for help where you got the server files from