SearchVarnish Enterprise
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 built into supported Varnish Enterprise, and does not require any additional installation steps.
The esi_limit parameter determines the maximum number of includes
(subrequests) in flight at each ESI level for a single delivery. The default
value of 10 provides a theoretical maximum of 50 simultaneous subrequests
with the default max_esi_depth limit of 5.