I'm running LostCities 4.0.7 beta. When setting the LostCities world gen options before world creation, there is a significant slowdown (delays of several seconds in between clicking a button and the click registering) and the Minecraft log file is flooded with:
Error adding element: Value 128 outside of range [0:64]
It looks like it's throwing the error about once every 2ms. I would upload my log file but trying to put it on Pastebin crashes my browser.

Same, lag is only on map-like screens (first and roads/tunnels).
1.16.5 Forge 36.1.3 LC 4.0.7 & 4.0.8.