Depends on Varnish version,
From Varnish 3.0.2 you can stream uncached content while it caches the full object.
https://www.varnish-software.com/blog/http-streaming-varnish
"Basically, his code lifts the limitations of the 3.0 release and allows Varnish to deliver the objects, while they are being fetched, to multiple clients."
The feature will be available on beresp.do_stream