varnish 5.0: varnish slow when backends do not respond?

[Hu Bert]

Hey there,

i hope i'm right here... i have the following setup to deliver images:

nginx: https -> forward request to varnish 5.0
if image is not in cache -> forward request to backend nginx
backend nginx: delivers file to varnish if found on harddisk
if backend nginx doesn't find: forward request to 2 backend tomcats to
calculate the desired image

The 2 backend tomcats do deliver another webapp (and are a varnish
backend as well); at the moment they're quite busy and stop working
due to heavy load (->restart), the result is that varnish sees/thinks
that the backends are sick. Somehow then even the cached images are
delivered after a quite long waiting period, e.g. a 5 KB image takes
more than 7 seconds.

Is this the normal behaviour that varnish does answer slowly if some
backends are sick?

If any other information is need i can provide the necessary stuff.

Thx in advance
Hubert