There's a problem in my server.When theres a large amount of people(about 21 humans),sometimes player will meet a situation like no more blocks to see more,no more blocks can be used,like lag,while my vpn do not have a crash.when players were lagging,I found that server's send out speed become very slow,cpu and memory had a low usage.(I set max-threads to 0,it means core will use all the theads)it might be a core problem.

when I stopped server and restart,players told me that something they did were eaten by server.when there are more than 15 people,server become lagging while low usage

when I was using 1.14.30 ver core,there arent that serious problem