I wonder what they'll be using for the authoring experience since Jekyll doesn't have any concept of browser-based backend for authoring and configuration. I guess removing that completely solves the difficult to log in to issue :)

Secondly, WordPress is 100% as performant as Jekyll or any other static HTML file if you put it behind a full page caching layer such as FastCGI cache in Nginx or mod_cache in Apache. Plus, you get some sane cache invalidation with plugins such as "Nginx Helper", "Nginx Cache Controller" and "NGINX Cache Optimizer".