You'd think there would be some sort of dependency build tree ala make/rake so only items whose source components would need to be rebuilt (which, in some cases could be the whole site).
And once you approach the level of a more typical WordPress site in both volume and information per page, I'm not sure how much effort you'd save. It's probably better to investigate how to make the general run time a bit faster. (Although it would probably be possible to have mostly static pages with few dependencies, then make the dynamic/dependent changes via a minimal web service and AJAX. At least when the load is too much for that, you'd still see the static content. Wouldn't be as generic/small as jekyll then, though.)