Scaling horizontally a Rails application where the bottleneck is the Ruby code is always very easy - just add more instances working on the same DB. It is usual

Scaling horizontally a Rails application where the bottleneck is the Ruby code is always very easy - just add more instances working on the same DB. It is usually more difficult because the bottleneck is in the DB, not the Ruby code. 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.