At VUG3 this parameter was discussed and the consensus was that 0.4

seconds was a bit too aggresive for a default value, and after some
talk forth and back 0.7 seconds was deemed more like it.

bin/varnishd/mgt_param.c

@@ -668,7 +668,7 @@ static const struct parspec input_parspec[] = {
 		"VCL can override this default value for each backend and "
 		"backend request.",
 		0,
-		"0.4", "s" },
+		"0.7", "s" },
 	{ "first_byte_timeout", tweak_timeout_double,
 		&master.first_byte_timeout,0, UINT_MAX,
 		"Default timeout for receiving first byte from backend. "