We’ve been working on adding parallel ESI in Varnish Plus for the last couple of months. The code is now feature-complete and waiting on more testing before being deemed production-ready.
The nicest thing about Varnish is VCL. You can define your own business logic in Varnish and have it run at more or less the speed of light. If you compare the performance of an interpreted language you’ll find that VCL will execute significantly faster as there is no interpreter that will dynamically glue various bits of binary code together.
Since the launch of Varnish Cache 1.0 - we've been asked about persisting data across restarts in Varnish Cache. Initially people asked about it not because it was needed but mostly because Squid, the software people were replacing, would retain content across restarts.
There is a term in Varnish Cache that every Varnish Cache user should know: “Hit for pass”. Like other Varnish Cache terms it is not self-explanatory and in order to understand what it is you’ll need to understand some of the mechanics of the caching.