Correct, but you posted in the ISPConfig 2 forum which implies that you are using ISPConfig 2. I'll move your post to the ISPConfig 3 forum.
Varnish does not support SSL but almost all websites today use SSL, so I won't use Varnish as a cache anymore. See: https://varnish-cache.org/docs/trunk/phk/ssl.html If you want to have a fast server, use nginx instead of Apache, nginx is supported natively by ISPConfig.
