The key to scaling Rails applications is effective use of caching (at multiple levels). For an online store (compared to, say, a live action game) so much of t

The key to scaling Rails applications is effective use of caching (at multiple levels). For an online store (compared to, say, a live action game) so much of the content that you serve will be cached that regardless of your apps runtime a lot of the user experience will be defined by how effective your caching strategy is. I think that Rails still offers enough compelling advantages for developer productivity to offset the (possibly) higher hosting costs. Although they probably spend most of their time on the application layer, I suspect the most important and trickiest work was done to scale the database.