The number of ads will only make a minimal difference. Ads normally slow down pages not necessarily because the script is taking time but because the image is being called from a remote place (a banner network) and thus is slower to reach you (if there is a bad connection) than the html from the site.

If you need banners (which will allows slow down a site) then adcycle does look like a good option but i would like to see some stats of some stress testing with links really.

http://www.ASciFi.com/ - The Science Fiction Portal