After 20s ... you have lost 50% of your visitors, because they think the damn thing is never going to finish loading on their connection and you will have triggered connection loss anxiety on multiple levels.

Also this is assuming, that one does not want to take all the things of that website and burn them after use, which is not a safe assumption to make on most websites, considering all the ads and trackers on today's typical websites.

An offline still working app sounds great. Unfortunately very few aspire to reach that point. Most already break to various degrees when you hit the back button.

Not every frontend is a marketing website where conversion is a concern. Marketing websites aren’t apps anyway, they shouldn’t have any backend - SSG is better than SSR for them.

Normally I'd agree about 20s being too long.

But I'm specifically talking about users on slow links. They often know they don't have a first class connection out, and have established a different set of expectations.

If your site is actually good enough to be one the users return to (and who here wants to build a site where that's not the case?), they all appreciate the site loading very fast the next time.