People who already know and love Varnish have been singing its praises for a long time. But we’ve seen over the course of the years that there are a lot of people who don’t know Varnish - even those who should, and who would definitely benefit from the content delivery performance boosts Varnish offers.
Varnish Cache 2.1 does what you can call store-and-forward proxying. It gets the request, then it turns around, gets the whole object from the backend, then turns around and delivers it to the client. If your backend takes time to deliver the object the client might get restless.