OMHMS server performance issue

Short info:

I have a private server for at least 3 years, and things are going ok, not awesome, not bad. Just ok.

 

The server is 'vanilla' with few cosmetic and admin plugins. Nothing, that changes the game mechanic and loads the server too much.

 

Heavy: AuthMe, CoreProtect, KeepChunk, WorldBorder, WorldEdit & WorldGuard (only for admins),

 

Light: InvRestore2, Itemizer (?), LuckPerms, Plan,

 

Pretty cosmetic: Action health, BetterBeds, elyby-skinsystem, ProtocolLib (for few functions), Vault, VaultChatFormatter, OpenInv, WorldEditSeelctionVisualizer.

 

Average online is 4-5 players, peaks are 13-15.

 

 

Detailed info:

Core: Spigot 1.15.2.

Even have tried paper, which for someone is "more efficient" but there is no difference for my case, all the same.

 

I'm renting a VPS with 4gb RAM and 1x vCore of 3.9/4.5 GHz AMD CPU with NVME SSD.

 

So.. looks like pretty good for small ~10 players server, right? But, Nah, that thing just starts lagging in some times and tps are going down for 5-10. That's terrible.

 

Moreover, I was thinking that the issue was in the server's hardware. Today's morning I order the higher VPS plan with 8gb ram and 2x vCores. And you know what? Almost nothing ate changed. Players still have freezes sometimes and tps in high-loaded chunks are still ~12.

 

We don't use any super-Redstone farms on the server. We have only one big thing... the huge castle which is made by players. There are about 1 million blocks was "mined" by players under the ground, and another million are above the ground. As I said, this is a huge castle. But it's the only one such a big structure. Other stuff like on any survival hardcore servers - some houses and little stuff.

 

When all players are outside that castle, TPS is about 15. When I fly to that castle, tps goes down to 11-13.

 

So..

 

Could anyone please guide me, what could I try to do to increase server performance? Because that's stupidly informal that 2x Core of 3.9/4.5 GHz CPU and 8 gigs of ram is not enough to be ok for 10 players...

 

Additional: I have a DB on that VPS too. AuthMe, CoreProtect, LuckPerms and Plan are using the DB.