Modpack VS tic-tacs

Starlight performs substantially better than phosphor and greatly speeds up terrain generation. Krypton for networking performance. C2ME or tic-tacs (both aim to do the same thing and are not mutually compatible) greatly improve world loading and terrain generation but are experimental and, in my experience, not compatible with a lot of other mods. Dimensional Threading puts each dimension on a separate thread, but also tends to not be very compatible with other mods. FerriteCore aims to reduce ram usage. This page has a whole list of mods.

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.

Tic-Tacs for world loading

I would recommend a fabric server running krypton, lithium, starlight, tic-tacs, hydrogen, and ferritecore. Those mods are geared towards optimizing chunk generation and loading while having 0 impact on vanilla game behavior (the last two are geared for reducing ram usage). Of all the performance mods I've messed around with, those are the ones that I would recommend most. You could also add carpet and set it up to add tps to the tab list if you want to easily see that number.

tic-tacs is an absolute must as it dramatically improves chunk load times

Also try Tic-tacs (can cause some issues).

And also run Tic-TACS which replaces Minecraft's clunky chunk loading engine for a multi-threaded awesomeness. This is the most experimental of the bunch, and the least recommended for any kinds of critical usage.

tic-tacs