What is also a major factor is when other players place chunk loaders in worlds I do not ask them too.
every world that is loaded aside from the overworld is a huge strain on the server.
Like right now there is 9 people on 20 tps and 1.5gigs of ram left. Yet there is 2 extra worlds loaded with no one on them. Now if the server got another 5 to 6 people the TPS would go down to low 14s, even if they were idle.