I tried to open http://mozilla.pettay.fi/html.pdf (html spec printed to .pdf), and pdf.js shows mostly just spinners in the pages. Some page may render for some time, but if you scroll away, and back, the spinner is often there again.
this is reproducible on all platforms
We changed around the rendering priority in the current nightly(17). You can still get spinners if you scroll very quickly, but since the rendering of pages with low priority is paused it feels much snappier. Try it out and let us know if we can close this.
Much better. thanks.
Verified fixed using Windows 7 and the latest Nightly.