Not to be a wise ass, but because I'm genuinely curious: who cares how fast it is? How often are you regenerating pages? Or is it a factor because of the change-make-check-change-again cycle?
It matters if it takes more than a short while to post your stuff. Jekyll/Octopress would take 20 minutes or so to regenerate my not all that large blog, I imagine that with 1000's of pages it would be unworkably slow.
I've tried Jekyll on multiple occasions and have always been put off by just how huge the tool is.
I just want to turn my markdown into pretty HTML. That's all. Jekyll feels like this giant sledgehammer that's built to hit a tiny nail... and doesn't even hit the tiny nail correctly every time.
In my case, when I wrote a plugin for Jekyll, I felt mildly upset about all Ruby things I needed to install and to some degree understand. I felt Jekyll was too complicated and complex. — Maybe if one is used to the Ruby world already, one won't feel in that way.