How to remove lag that lasts 2 minutes on my server?

My server is working properly and I allocated enough RAM on it (4GB) but as soon as my friends join they like for about 2 minutes then unlag afterwards, and if they die they lag for 2 minutes again!
Please help!

Added (1). Oh sorry, the game is Minecraft

Well it may help if you tell us what game this is. I'm going to assume it is minecraft. And if it is, you could try removing the nether and lowering the number of players allowed.

I have problem as you, I searched on google on few days and I find out a successful method to fix this problem.

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