Varnish Not Cache When Cookie EXTERNAL_NO_CACHE added

[Infotech-307]

Hai, I have issue with magento varrnish caching, and found this extension for manage it. but after I install and try the caching performance, I found issue while some cookie show on magento.

1. while I see caching performance on magento home its find, there is cookie called "frontend" show and its handled, page document load 8ms its very fast.
2. I go to product category page and its also work.
3. I go to product vview page and its also work with new cookies "external_no_cache, PAGECHACHE_FORMKEY".
4. but after I add product to cart, the page performance go down became 800ms, and after that, in all page mageto, load page became above 700ms cache varnish pass, not work.
5. I try see whats the problem and found the new cookie called "EXTERNAL_NO_CACHE", affter I remove this cookie page load run normal again with page load 8ms varnish cache work.
6. I try to create cookie with "NO_CACHE" word, and page not caching, page load became increase above 500ms.

so whats the problem?, its because thats cookie all varnish cache not work?. or because session product cart.? please help me to solve this. thanks.