Minecraft server is lagging despite 20tps?

I've built a server on which I want to play Minecraft with a friend.

However, it was WITH and WITHOUT mods with 20tps. The TPS sometimes drops enormously to 7 to 4 but it was still there.

Xenon E-2144 4-core 3.60Ghz

16 RAM memory

GPU- Gtx 560 Ti 256mb

Forge server 1.12.2 14.23.5.2855

I ask for help.

How much RAM did you give the server?

I've tried from 1GB to 12GB which was the same across all.