Really, its not about the Immunization or cookies that might slow down or hasten your bandwidth or speed of your connection. Cookies are just files placed in your browser when you visit a site. It's more like a file, that will "remember" your visit to the site. Some people and programs consider cookies malicious as they track the user's daily activities... like the sites the user visits. I usually clear my cache, temp. files, and cookies from my browsers after every session. I get around 30-50MB of trash

Immunization basically, adds "killbits" to prevent some hundreds of cookies from added to your browsers and ActiveX's. The Immunize feature also adds "bad sites" to your restricted settings on your browser.

http://forums.spybot.info/showthread.php?t=123

This will have some details about what Spybot immunizes.

Clearing your cache, temp. files, and cookies (Private Data in Firefox) might speed things up. It may also help to defragment your drive every two or three weeks. If you have some chunk of cookies and temp. files in your browser, it may act slow.