Use Cookie-free Domains for Components
This is an excerpt from Yahoo Best Practices for Speeding Up Your Web Site
When the browser makes a request for a static image and sends cookies together with the request, the server doesn’t have any use for those cookies. So they only create network traffic for no good reason. You should make sure static components are requested with cookie-free requests. Create a subdomain and host all your static components there.
If your domain is
http://www.example.org, you can host your static components on
static.example.org. However, if you’ve already set cookies on the top-level domain
example.org as opposed to
http://www.example.org, then all the requests to
static.example.org will include those cookies. In this case, you can buy a whole new domain, host your static components there, and keep this domain cookie-free. Yahoo! uses
yimg.com, YouTube uses
ytimg.com, Amazon uses
images-amazon.com and so on.
Another benefit of hosting static components on a cookie-free domain is that some proxies might refuse to cache the components that are requested with cookies. On a related note, if you wonder if you should use example.org or http://www.example.org for your home page, consider the cookie impact. Omitting www leaves you no choice but to write cookies to
*.example.org, so for performance reasons it’s best to use the www subdomain and write the cookies to that subdomain.
Ref: Please read here