The version 1.12.2-r152 of Actually Additions mod linked bellow causes cascading worldgen lag, by itself it wouldn't be that bad but in any sizeable modpack it adds on quite a bit of cascading worldgen lag. I have also included a link explaing exactly what that is and how to go about fixing it. Example setting an offset or flags when structures are spawned partially in a chunk and when adjesent chunks load it reads it and finishes generating the structure.

https://www.reddit.com/r/feedthebeast/comments/5x0twz/investigating_extreme_worldgen_lag/

https://www.curseforge.com/minecraft/mc-mods/actually-additions/files/3117927