Beg to differ with those that say the Adsense ads do not slow down a site.
a) You guys evidently didn't try to surf the day Google services were down.

b) I surf a lot and when a site is slow loading a lot of the time it is the 3rd party scripts that are slowing the loading. You can tell what is holding up the load by looking in the browser status bar in the bottom right corner of the browser window (if you have it turned on).
Solution:
Put the scripts at the end of your web page coding and manipulate it's location in your content with CSS. This way the rest of the page content can continue to load for your visitors to see so they don't leave and your ads/3rd party stuff will load later.