So are web applications that are completely behind auth and never need to care about SEO.

They're quite different and it's a shame they're usually mixed up in these discussions.

Many sites have a two tier application with a CMS SEO “landing page” abd conversion funnel run by the marketing department and a user-login that takes you to a separate website that is SEO irrelevant.

So… a site that doesnt care about SEO and is not indexed and is purely for logged in users is also very common.

I don’t really understand the disagreement here. I didn’t claim those sites are rare. I was specifically refuting the OPs claim that page load time only matters in a small number of specialized domains. I am saying it is very common for it to matter. It’s also very common for it to not matter, you’ll get no argument from me there.