I agree that Varnish might be a good addition but in the end it depends on the exact setup you're running and which hardware resource is used the most. My opin

I agree that Varnish might be a good addition but in the end it depends on the exact setup you're running and which hardware resource is used the most. My opinion is that the CPU is usually the first to fall and you will not help it quite that much with an in-memory cache when the actual problem is the https overhead or the PHP/DB processing.