I created a customized world in 1.12 long ago.  I have since updated it to 1.13, 1.14, 1.15, and 1.16.  In 1.16 I decided I wanted to remove the "customized" designation of my world as it was a bit of a pain every time I updated.  So I ended up creating a new world and copying my builds into it.  They are large kingdoms occupying huge areas.  Every kingdom is just fine with no issues, however there is a problem with one of them.  Whenever I'm in the area, my FPS will drop to about 10 or so after a few minutes of moving around.  This doesn't happen in any other kingdoms.  This one also has the least amount of items (chests, frames, etc..) in it.  It's mostly trees with a few small houses in it.  In the minecraft menu I went to "single player", then selected my world and hit "edit" then "optimize" hoping that that would clear up the issue but it hasn't.  I don't know why all my other kingdoms (which are larger) do not have this issue.  It's just this one area.  Please help.
Thanks
Klautos