Just to remain slightly on topic, you can't really compare an entire application stack like Rails with a language like Go. Maybe you could do with something lighter like Sinatra.
For me Rails big win isn't that it's high performance (although done right it can handle a decent volume of traffic), but the fact it's configured out of the box. You can be productive on day one, rather than wiring together low level libraries for database access and the like.
Finally having said all that I'm having a really good time with Pliny, Heroku's Ruby API stack, which feels like I imagine Rails might have done if it were built specifically for REST APIs rather than full applications with a UI.
One of my big gripes with Rails is defaulting to ActiveRecord, compared to pretty every alternative (my favourite is Sequel). This is the problem with Rails: You can be productive if you can be productive with its default choices.
But in every project where Rails has been involved that I've worked on, sooner or later people start fighting one of the Rails default choices, and the problem with that is that so many parts are expected to be there by other components they bring in, so even things that are in theory replaceable are in practice tightly coupled.