Ever used the WP-API? It is a REST API and you can bypass the native templating system and even the WP admin entirely. There are plenty of people are building web apps with WP/WP-API as the back-end. WordPress is a lot more than it used to be.

The Wordpress backend still has scaling issues with many popular plugins. Helping a site with 100,000 customers in a woocommerce database migrate to a more robust ecommerce solution because it takes 15 minutes to log in currently.

Last time I used the API it was incomplete and slow. The "multi request mode" also exposes all sorts of resource exhuastion attacks. I had a ton of sites go down from "bundled" login attempts where they tried something like 100 passwords at once. I usually end up blocking it completely

For a while I did something similar. I used a private instance of WP to generate my blog content and then scraped the site to get static pages which I could then serve from another machine. That meant nothing dynamic, no WP "vuln of the week" treadmill, and no cache-layer validation headaches either. It worked pretty well. For a larger site I could have made the generation more selective, and (re)optimized the output in many different ways, but it wasn't worth it. Then I got off WP entirely and it was all a moot point, but it's an approach to consider for those stuck in WP-land.

I'm curious what does a "medium traffic site" mean? I feel like I have a bunch of what I'd call "medium traffic sites" and I've not had much of a problem with them. Though, maybe my "medium" is actually "small"?

Is there a general rule of thumb for site traffic? Like a "medium traffic site" gets X Hits, or whatever?

100 requests / sec average seems medium traffic to me (and outside the bounds of wordpress live)

8.6 million requests/day is only medium traffic?

Sure. Well let's call it 1 million per day peaking at 100 req/sec... not many websites are steady state 24/7

But this is orders of magnitude below what a Google or Espn or Amazon do. But orders of magnitude over what 1 PHP server can do. (But oddly enough, within the realm of possibility for a good Java or Go or Rust server)

Nonsense.

I just ran a load test against WP yesterday and with a traffic load (from tsung cluster) peaking at 430 req/sec, it was easily served by 5 t2.micros running Varnish + WP (behind Apache no less) + memcached.

The load test included random search strings 50% of the time so half the tsung visitors were getting live service from Wordpress.

Plenty of sites would like to disagree with you.

Nginx and PHP7.1-FPM will make it quite fast, even on a $5 DO server. Caching will speed it up quite a lot more, but you should always have your pages behind a cache, thats one of the points of a CMS is to generate you html that is stored until modification.

I don't agree that WordPress is as bad as your saying... But the static HTML caching part can be easily achieved, with plugins or with Cloudflare for instance.