Behind the scenes it's just running the usual packages for templates, markdown, minification etc., so for a fresh build it's about as fast as any typical grunt-based setup. But AFAICT it correctly detects when things don't need to be regenerated, so incremental builds are quite fast no matter what, unless you're editing templates.
I recently converted my wordpress blog to hexo and blogged about it, if you're interested:
I wanted the same, ie speed with ease of templating, so I made Griffin[1] which uses handlebars templates and can parse 5000 markdown pages in ~8 seconds on my machine.
It's not blazingly fast but it's good enough for me - a blog with about 50 posts takes about 30 seconds to deploy from a clean start. It does sensible things like caching the json database and only generating new posts.
I recently converted my wordpress blog to hexo and blogged about it, if you're interested:
http://aphall.com/2016/01/migrating-wordpress-to-static/