Agree. Ruby tends to be more likely to be CPU-bound than other languages. That demands a more indepth understanding of Ruby itself. For example, as simple as string concatenation of use '+' vs '<<' can easily fail lots of people.

Yours is the typical answer from rails fans.

I don't quite like the tone of this statement. Although I've been working with Rails at work for the past five years, I don't consider myself a Rails fanboy. I consider it a valuable tool to bring along high productivity. I am quite aware of its strength and weakness.

But what could be wrong with his Rails implementation that could be that slow, assuming they are reasonably competent - a valid assumption I think given their successful port to node?

We all know that scaling up Rails is doable though not an easy task. To handle webscale traffic like LinkedIn in Rails, you can not go very far following the traditional ActiveRecord way. Very likely, you will introduce a high performance middleware to prepare the data for rendering say in Java/C/C#. For the frontend, you have jQuery or whatever Javascript library you like. You just leave Rails to handle routings. Though some people may challenge that if it is necessary to have Rails in this architecture. I think Rails can still make a case even in such scenarios, which is basically what github does.