Log excerpt:

[16:47:29] [Server thread/WARN] [FML]: Recurrent Complex loaded a new chunk [1, 2] in dimension 3 (Flatlands) while populating chunk [-95, 121], causing cascading worldgen lag.
[16:47:33] [Server thread/WARN] [FML]: Recurrent Complex loaded a new chunk [0, 3] in dimension 3 (Flatlands) while populating chunk [-95, 121], causing cascading worldgen lag.
[16:47:37] [Server thread/WARN] [FML]: Recurrent Complex loaded a new chunk [5, 3] in dimension 3 (Flatlands) while populating chunk [-95, 121], causing cascading worldgen lag.

What I wish to draw your attention to is that Recurrent Complex loaded chunks [1, 2], [0, 3], and [5, 3] while populating chunk [-95, 121]. [-95, 121] is nowhere near any of the chunks that Recurrent Complex loaded. This seems very wrong to me, and I suspect is the cause of some of the "half-generated" buildings I'm finding in this world. It also causes significant worldgen lag.

While this particular instance is in the Flatlands dimension (we are using OTG), I've also seen this for all other dimensions from time to time.

Minecraft 1.12.2; Forge 14.23.5.2836; RecComplex 1.4.8.2; OTG v6; ChunkPregenerator V1.12-2.1

COOL

Helpful for the Modder: https://www.minecraftforge.net/forum/topic/60721-1121-how-would-i-prevent-cascading-worldgen-lag-during-worldgen/ (see Post by Draco18s Posted September 14, 2017)

Or keeping it short:

I have worked with several mods over the last couple weeks to fix these worldgen issues.
The solution is often very simple, they just need an offset. However the explanation is not always clear, which is why I made this post.

Having the same issue btw. on v.1.4.8.2

I think I'm seeing this same thing on my end. Although in my case it's not even generating chunks, just while I'm wandering.

[14:30:55] [Server thread/WARN] [FML]: Recurrent Complex loaded a new chunk [76, 57] in dimension 0 (overworld) while populating chunk [76, 56], causing cascading worldgen lag. [14:30:55] [Server thread/WARN] [FML]: Please report this to the mod's issue tracker. This log can be disabled in the Forge config. [14:30:55] [Server thread/WARN] [FML]: Recurrent Complex loaded a new chunk [77, 61] in dimension 0 (overworld) while populating chunk [76, 62], causing cascading worldgen lag. [14:30:55] [Server thread/WARN] [FML]: Please report this to the mod's issue tracker. This log can be disabled in the Forge config. [14:30:56] [Server thread/WARN] [FML]: Recurrent Complex loaded a new chunk [77, 62] in dimension 0 (overworld) while populating chunk [76, 62], causing cascading worldgen lag. [14:30:56] [Server thread/WARN] [FML]: Please report this to the mod's issue tracker. This log can be disabled in the Forge config. [14:30:59] [Server thread/WARN] [FML]: Recurrent Complex loaded a new chunk [70, 54] in dimension 0 (overworld) while populating chunk [69, 54], causing cascading worldgen lag. [14:30:59] [Server thread/WARN] [FML]: Please report this to the mod's issue tracker. This log can be disabled in the Forge config. [14:30:59] [Server thread/WARN] [FML]: Recurrent Complex loaded a new chunk [73, 53] in dimension 0 (overworld) while populating chunk [73, 54], causing cascading worldgen lag. [14:30:59] [Server thread/WARN] [FML]: Please report this to the mod's issue tracker. This log can be disabled in the Forge config.