Here’s an interesting article by Stefan Baumgartner on using Jekyll for larger sites. I’ve noticed myself that the build time for this (Jekyll based) website takes a lot longer with a large number of posts.
Generating this website with just the blog takes a few milliseconds, but when I add my concert photo galleries it bumps up to 6 seconds. Still fast enough, but I can imagine this becoming a problem at a larger scale.
His solution (splitting up the different parts of the website) is interesting, simple, and very effective.