I took a look at this one too and on the pages where FA was not showing I could not quickly locate the call to FA resources. I didn’t spend all that much time on it, but I presume you do know where it is|was because you categorised adding the call in the document head (which I was going to suggest too) as the ‘second’ call to FA.
Was the first call embedded in a CSS file someplace that isn’t quite as obvious? Or some other technique? If that first call wasn’t ‘working’, in the sense that the icons weren’t actually showing up, does that first, non-effective, call to FA actually count at all?
Wouldn’t it rather be that this first ‘faulty’ call to FA would be responsible for any slowdown as that call would presumably have to 4xx out? Not the second which, from reading the latest posts, seemed to successfully bring the FA resources into scope?
Sorry to ask so many questions! But if you feel like educating us it would be great to benefit from your knowledge/interpretation further.