Traditionally, all ESI includes in Varnish were fetched in sequential order, one after the other as they are required for delivery.
The improved ESI delivery implementation will seek out all the include fragments and issue backend fetches for all of them concurrently, which in turn significantly reduces the load times for ESI content.
Parallel ESI is available in Varnish Enterprise
4.1.4r5 and later.
Parallel ESI is built into supported Varnish Enterprise versions, and does not require any additional installation steps.
The parallel fetch behavior replaces the previous sequential fetch implementation. No parameter settings are required in order to enable it.