One thing that can slow sites way down is web trackers. You can see this for yourself with the Firefox and Safari Ad-on Ghostery.
http://www.ghostery.com/Many sites even use several trackers and Ad platforms and this can slow them way down. We use no web trackers and only one Ad platform, Google Adsense and this is only for none premium members. We do this to keep the site fast and responsive and we don't want to track our users. Others sites like Gizmodo, MSN, Engaget can use up to 16 or more. Even MS has 5 on their main site. This seems strange as most commercial corporate company sites have zero.
You might want to grab Ghostery and have a look around. You might just be surprised what you find and this Ad-on gives you some control over how sites track you.
