This implies that the typical SPA, once loaded, is just "there" and can be used rapidly as if it's some offline app. In reality, many of them make a huge amount of fresh network calls on each navigation.

Still, it's an interesting question from the user's perspective. Reminds me of the downside of lazily-loaded images on the web. Whilst designed to help people with data constraints, it can have surprising side effects. The anecdote that comes to mind was a rural town in India where inhabitants would visit a particular place for internet access. They'd open a few URLs and just let it download, could take some 20 mins. Next, they'd return to their home to actually look at the pages they downloaded.

This usage pattern doesn't work with lazily loaded images, it forces them to scroll and wait beginning to end on every single URL.

It depends on where those 15 seconds fit. If I have to hold onto some thought for 15 seconds, forget it. If I'm loading an IDE or some other tool I'll use all day, it's fine.

My bookkeeping software has lots of 1 second delays and it's really annoying.