I'm not sure if this is exactly where al2o3cr was going, but a Ruby (or whatever) application would normally include such things as caching and queueing. Despite the slowness of Ruby et al, I actually would expect a modern application with these layers to handle load much better than straight database calls. So I guess I would consider the PostgREST implementation a bottleneck because you can't add any of these layers.

Why couldn't you add a plain HTTP caching layer (e.g. Varnish) on top of PostgREST?

Yes, you could. My only point is that PostgREST limits you in what you can do in terms of handling load.