This is pretty interesting because the extends certainly result in DRYer CSS, which I think most of us would have figured to perform better.

I know I've gotten myself into some hairy situations in the past by mistakenly extending things that were already extended, and unintended consequences are a concern even now.

I've got a hefty redesign due to launch in the near future, and I'm doing a lot of performance tuning. I've got ~300 includes (mostly media queries) and ~30 extends. This seems like an easy enough change to make right now for a quick, but small win.