Tuinity VS Akarin

Could also pregenerate your world with chunky. To make it quicker add Starlight, Lithium, tic-tacs(This might be incompatible with some worldgen mods, but it massively speeds up terrain gen speeds if it works), and Carpet(use the "/tick freeze" command to freeze the game so there's more resources for terrain gen). Then switch to a paper style Tuinity server and set the worldborder to keep people from moving past pregenerated chunks.

You can try out Tuninty for extra performance

Tunity - Github Repo - (no website)

Strip out all plugins, make backups of & reset your yml's. Does the bug still occur after doing that? Then report on their issues tracker: https://github.com/Spottedleaf/Tuinity/issues

Tunity: https://github.com/Spottedleaf/Tuinity

Tuinity is a fork of Paper that includes optimizations that are generally tailored for servers with high player counts. It also includes Starlight: a complete rewrite of the lighting engine which greatly improves the speed of chunk generation. One benchmark has shown that Starlight can generate light 35x faster than vanilla.

Tuinity

For server jar I would recommend using Tuinity (or maybe even Purpur). As Tuinity has some more optimizations that help for larger servers and Purpur is a fork of it that I believe has even more experimental optimizations.

https://github.com/Spottedleaf/Tuinity/blob/master/patches/server/0061-Rewrite-the-light-engine.patch here is the patch, at the top he writes a description that can help you read more about it.

Use Tuinity (it's a fork of Paper, optimized for higher play counts)

For 1.12.2 try using Akarin. Its a super optimised, (mostly "multicore") jar. And it even works with bukkit plugins!

" The situation with third-party software is that while additional work (from plugins) do use additional cores, the core world mechanics (ticking, etc.) largely remain the same. Attempts to make the core mechanics multithreaded include Akarin, but do be warned this is very complex work and things get buggy very easily."