I doubt you can get statistics on that… You may have heard stories about visa applications taking so long that the trip became pointless, e.g. granting visa for

I doubt you can get statistics on that… You may have heard stories about visa applications taking so long that the trip became pointless, e.g. granting visa for travel to a conference on the last day of the conference? That's a "yes" in the statistics, a granted application. And often there isn't any data at all. I've been questioned by immigration for ten minutes a couple of times. No record was made as far as I could tell. I suspect that if a country wants to, people can be kept for quite a while without any record being made. So if you want to know whether the US is moving from a top-25% position to something rather worse, I fear you might not be able to get enough reliable data for a comparison.