Acrobat gives you a choice of compression levels. Just like a web page, if the images themselves are not compressed or optimized for the web, it's going to take a long time to load. For a web-based publication like you're talking about there's no reason the images should be above 72dpi unless you want people to be able to print the stuff. I just did a banner for a company (a real banner, not a web ad) that was to be printed out at 200dpi 12"x76" and it only took up 2.5mb, zipped; call me the KompressionMeister.