In reply to Grant Keller.

The situation with performance is not that straightforward or linear. Loading WP core is relatively heavy operation. If you manage to switch context in a single API request then it comes ahead.

But for any use case which takes multiple requests (and there is often way more than one thing going on a WP page) the cost of spinning up multiple core loads will add up very quickly.

In my opinion current REST hype overblows performance benefits for content sites, while downplaying development and maintenance costs (like doubling up of templates in this case).