Factorio isn't multithreaded, so with a quad core the maximum percentage cpu used is 25%.

Huh, so the CPU is bottlenecking this? Is there a way to sidestep this at all? As mentioned I already tried dropping graphics settings to minimal but it didn't do much. Also saw some people mentioning ...

First off, huge fan of the game, I have about 250 hours in so far and this is the first real issue that's occurred. I'm currently running my first decent-sized factory in an effort to knock out the mass production 3 achievement; nothing compared to some of the massive bases I've seen here, but still ...