> Will this cause performance issues for sites that use static cookieless domains for js, images etc
> Google themselves do this with gstatic.net and ytimg.com etc
Most probably not. The point of cookieless domains is that you can use a very simple web server to serve content (no need to handle user sessions, files are pre-compresses and cached, etc.) and it lowers incoming bandwidth a lot. If you have a lot of requests (images, css, js) the cookie information adds up quickly.
Opening video thumbnails from ytimg.com will still be cached for youtube.com as before. The only thing that will change is for embedded videos on 3rd party websites as those won't be able to use caches ytimg.com thumbails from elsewhere.
> Google themselves do this with gstatic.net and ytimg.com etc
Most probably not. The point of cookieless domains is that you can use a very simple web server to serve content (no need to handle user sessions, files are pre-compresses and cached, etc.) and it lowers incoming bandwidth a lot. If you have a lot of requests (images, css, js) the cookie information adds up quickly.
Opening video thumbnails from ytimg.com will still be cached for youtube.com as before. The only thing that will change is for embedded videos on 3rd party websites as those won't be able to use caches ytimg.com thumbails from elsewhere.