I think there are different use cases, though; like, I agree with you that I find it really really annoying when I'm starting to read a paragraph of text... but

I think there are different use cases, though; like, I agree with you that I find it really really annoying when I'm starting to read a paragraph of text... but, for the various buttons and navigation structure--even the headings--having them temporarily in the wrong font is a major win, as it lets me start to orient myself into the page (and might even help me get to my next nav point and out of the site before the font even bothers to load).