For example, going to the database and retrieving the data uses less CPU and time than deserializing the data into an object into Ruby. That is the sense I am talking about. The bottleneck becomes the code talking to the database rather than the database itself.

That's...that's not how it works. I was talking about the scalability of the database as that's a brick wall you hit long before anything else. You can always fire up more rails workers, scaling persistence is a PITA.

Are you intentionally misunderstanding me in order to make a strange and irrelevant point?

I'm not making a pure scalability argument. Your solution of scaling the rails tier still suffers from far higher latency thus making choice of language/VM relevant. Rails is very, very slow — even if you cache everything behind it. So yes, you can scale it up, but it is at least 10x more expensive to do so.