That is a very good point, but it's not the argument here: I'm responding specifically to the idea that webserver and webserver+fastcgi are "technically" the sa

That is a very good point, but it's not the argument here: I'm responding specifically to the idea that webserver and webserver+fastcgi are "technically" the same speed. Running two web servers (one speaking HTTP and one speaking FastCGI) is necessarily going to be slower than running one web server. This should be obvious, although it might be "not significantly slower", which is why I provided some real numbers from my experience to show at which point it becomes slower by an order of magnitude. You might also find that it's easier to debug one webserver than two.