While running mod this appears in log [Server thread/WARN] [FML]: Primitive Mobs loaded a new chunk (-16, 23 Dimension: 0) during chunk population, causing cascading worldgen lag. Please report this to the mod's issue tracker. This log can be disabled in the Forge config.
I think it's coming from here
More info here: https://www.reddit.com/r/feedthebeast/comments/5x0twz/investigating_extreme_worldgen_lag/
Mod version 1.1.5b
Forge 2590

Thanks for reporting. Wondering why this happens. The presented code does what the reddit post says. It takes the chunk coordinate and adds a random int between 0 and 16 to that coordinate; then it loads the Mimic chest. I'm having trouble seeing how it could go into the negatives or over 16.

Here few examples how it was fixed in other mods: 1 and 2
Maybe it will give you a clue.

Thanks!

I did the same thing so hopefully this is now fixed for 1.1.5c-beta which is processing for upload.