Performance matters rarely in my experience. There is a world of low load services out there, with servers sitting idle most of the time waiting for requests. And that's not the case of Shopify which is doing well with Ruby anyway.
It's also very easy to scale horizontally the execution of the web code. That's why, for most web software, programmer productivity is more important than raw language performance
You're right about price though; if the cost of compute is relevant in your business model - which, again, usually isn't the case - then doing that computation with Ruby on AWS isn't the best choice.
Surprise bills from AWS means it is time for refactoring ( and thus optimization time ;)