I am experiencing this too, quite frequently (for a few hours a few days a week). The culprit is
ads.stackoverflow.com in 90% of the cases and
gravatar.com in 10%. It's especially annoying because before ads has not loaded, you can't start an answer or write a comment. I'm located in Germany, on a 100 Mbit DSL line in the office, and on a smaller DSL line at home, both with completely different providers. I have had this problem on both lines. In both cases, I'm behind a standard router, but that's it - no Proxies or anything.
I guess some downtime, lag or routing problems can't be helped, but it would be a great help if the page could be designed in a way that you can at least already start typing even if not everything has loaded yet.