Is content accessed through a click-through link cacheable by Varnish or similar?
So, only the civicrm click-through requests are grinding the VM/Server?

I am not seeiing how to cache those without breaking the counter functionality.

Is site OK when not using the click-through functionality? To make 100% sure that those are the cause, and not some other Varnish config (like not caching when certain session cookies are available).

If it is really the CiviCRM counter functionality and not the pages it forwards to and you need that functionality then I only see 2 options:


upgrade the server/VM
send the mailing in batches (maybe even spread over a few days)