Exactly. I've also been involved with read-heavy Rails sites serving well in excess of 1000reqs/sec, and I know that others sites (Shopify et al) do way more. A

Exactly. I've also been involved with read-heavy Rails sites serving well in excess of 1000reqs/sec, and I know that others sites (Shopify et al) do way more. As you point out, it's all about having an intelligent caching strategy and extensive use of CDNs. Most scaling issues I've seen on well-designed Rails sites have been to do with the DB, not the framework!