Great that it's working for them, but for the end user the feel of the site nearly unusable. There's 1s+ of delay on every interaction - pressing "home" button from explore tab takes 1,85 seconds (on a gigabit connection) before home view comes active, without any other feedback to the end user except for "Home" icon becoming active.

You cannot just blindly trust the page speed metric but it should be impossible to miss things like this when you are actually using the site. Compare the experience to something like GoodReads that's using plain old SSR and you'll immediately notice the difference.