Most banner ads are of predetermined size, displayed using <IMG> tags with size
attributes, so page rendering can be done before the image has been downloaded.
So, the user should be able to see the content right away, even if the ads take
a while to fill in.
Sites whose ads don't work this way are broken. HTML ads, of course, can't be
fixed this way, so if your site is pulling HTML ads from an ad server, it
should time out aggressively when the ad server doesn't respond.