In reply to Rarst.

This is exactly the main issue with using REST for this purpose.

Wallace theme is very simple, so it shows one post and runs a single REST request.

What happens on a typical page that has widgets, related posts, comments? The number of REST requests can be 10, 15 or much more. Each request will run a full instance of WordPress. So, to generate a page using REST, instead of a single WordPress instance for normal PHP theme, REST theme will run 10 or more WordPress instances, taking significantly more system resources than normal PHP theme.

REST is a great tool, but using it to fully replace PHP theme with JavaScript only theme will increase server usage tenfold for typical websites.

REST call is not some magical unicorn that will solve speed problems just like that, it is server performance killer if not used properly.