I haven't been able to see it even setting Chrome developer tools to crazy slow network speeds. Maybe chrome developer tools simulation isn't enough to see it?

The observations you are reporting are from images you created yourself and you know are in fact progressive jpegs? (not "regressive", although that's funny). There are of course other techniques to start with a lower resolution image that do "work" (in the sense of actually displaying a lower resolution image first at least).

I deliberately was talking about decode speed. The question is if you serve even via moderately fast infra, does it display faster? In my case on a (indeed fast) local system absolutely not. Mere size can be a decode problem of course. But it's extremely hard to tell that a single digit percent size difference is an advantage for serving.

But if better compression for storage or you can verify progressive serves faster then it is of course a benefit.

I guess the point I am making is that most people think: I heard it's somehow better so lets use it.

I've started using computers in the 90s, I've seen this every day back then.

Still the question is, does it help? Trying to access an average web app will probably take minutes before the browser may even see an image. If you do everything possible to render reasonably fast on very slow speeds, then progressive is nice. On a fast connection I don't think the average user will notice the difference.