[Tobu]

Another reason is: because these cdn domains aren't the domain people navigate to, they don't get cookies from the domain that includes them. Cookies bloat requests for the domain they are assigned to; sending them only once per page is faster.

[gbhn]

As I recall, for Google Video Search, we used domains like "1.vgc1.com" "2.vgc2.com" etc for cookieless hosting. A short domain name (as opposed to 'cookieless.googleserving.googlevideo.com' or some such) saves bytes in the HTML, and cookieless domains save bytes in requests, as well as providing better cache hit rates and such. Multiplexing domains lets the browser initiate several simultaneous requests for scripts, images, css, etc. (I think this is less of a problem these days, though.)

Some of these problems are addressed by modern browsers and other techniques, but getting good performance out of the median web browser remains a big challenge.

[enneff]

Unnecessary cookies can also defeat caching in some instances.