I had some success in loading Mozilla's JS library asynchronously (using RequireJS in that particular application) after initial render/dom-load. That certainly mitigates some of the performance issues. (You'd notice even more of a bit of a slowness in the UI picking up if you were already logged in on page load, but that is more and more common on the internet these days, for many of the "SSO" authentication systems, so I don't consider that much of a problem.)
Also, from your waterfall you've posted here, most of the Persona stuff is happening in background/parallel anyway: the big slowness in getting to render start certainly appears to me to be your fonts more so than Persona, which is what I would expect to see. That is, I'm wondering if you are scapegoating a bit here as the impression that I get looking at your waterfall is that you won't gain as many milliseconds as you might think by removing Persona.
Also, from your waterfall you've posted here, most of the Persona stuff is happening in background/parallel anyway: the big slowness in getting to render start certainly appears to me to be your fonts more so than Persona, which is what I would expect to see. That is, I'm wondering if you are scapegoating a bit here as the impression that I get looking at your waterfall is that you won't gain as many milliseconds as you might think by removing Persona.