You clearly don't understand the problem. Even mod_pagespeed or memcached would be more appropriate here: They are rate-limited by the LISP kernel anyway (we are talking about dynamic content here).
Varnish sits in front of the backend, responding to all requests that it has cached content for directly, without bothering the backend at all. It lives higher in the stack than than pagespeed or memcache, and not limited by the backend's speed in any way.