My guess? It's doing streaming parsing/processing, so it's network bound. It started downloading html, once it got the first byte it started processing it, but

My guess? It's doing streaming parsing/processing, so it's network bound. It started downloading html, once it got the first byte it started processing it, but then it had to wait for the rest of the bytes (not to mention the css file to download). The parent comment is clearly using some really slow wifi, so I think it's likely that's what happened.