From Doctor:

nether terrain gem seems to be taking much longer with quark, especially after using a soul bead which seems to absolutely kill the terrain gen
tested it in an instance with just forge and it seems to be fine, pretty much instant generation

I'm not sure exactly what's causing this yet.

Took a few spark profiles (with --thread *), I was expecting quark's custom worldgen code sticking out like a sore thumb but nothing really caught my eye. Hmm.