edge-side-includes

Im trying to use esi to make a ninja caching on my site. The idea is, the site is mostly static, I just need to do fancy stuff if the user is logged in or not. So I was trying to put an on the page A, and set triggers in the application at page B. This way I could cache the page A on varnish, and let the server deal with the small work that is page B. But the cookies I've seted on page B were not forwarded to headers of page A and didn't work =/ Is this that Im trying to do possible? I could use ajax, but doing this inside the server, before sending the page to the user seems more correct to

Im wondering how the performance of th ESI module is nowadays? I've read some posts on the web that ESI performance on varnish were actually slower than the real thing. Say i had a page with over 3500 esi includes, how would this perform? is esi designed for such usage? We're using Varnish and ESI to embed sub-documents into JSON documents. Basically a response from our app-server looks like this: [ <esi:include src="/station/best_of_80s" />, <esi:include src="/station/herrmerktradio" />, <esi:include src="/station/bluesclub" />, <esi:include src="/station/jazzloft" />, <esi:include src="