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:

http://aphall.com/2016/01/migrating-wordpress-to-static/