A monolith doesn't necessarily mean slower, even with Ruby. There are lots of opportunities to run less code on each request, do some work with the db, and to split off measure bottlenecks into services in a faster language. It's often good to build things quickly, find fit, and then carefully measure before you introduce calls over the network.
A monolith is a problem which they are also breaking down, when you want to make something faster it's easier to target the service that does it vs the giant app.
Edit: getting downvoted by people that don't work at Shopify.
