(Shameless copy of @mezz issue reported at other mods) --Edit-- Sorry, didn't mean to ping you --/Edit--

In Forge 1.11.2 - 13.20.0.2258 there is a new detection for worldgen features that load new chunks and cause more worldgen.
This can cause a lot of lag when loading new chunks. Mezz wrote a post about the issue here](https://www.reddit.com/r/feedthebeast/comments/5x0twz/investigating_extreme_worldgen_lag/).

Mekanism loaded a new chunk (-17, 16 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.

You can debug this by grabbing the latest Forge, creating a new world or flying around, and putting a breakpoint at this log (at the bottom of Chunk.java). You should see that one of your worldgen features is loading a new chunk and causing more worldgen.