The first PDF.js document uses the DOM instead of Canvas like the previous one. This is done to support text selection. Most of the time is spent in the style system. I don't know that area very well but in the past I've seen simplifying CSS selectors make all the difference. I know a fairly important problem for B2G is spending up expensive style flush (https://bugzilla.mozilla.org/show_bug.cgi?id=931668) but I don't know enough about CSS to know if that fix will solve the problem here.