As someone who spent a few years in the codebase for Tracker. There was some attempt at “Clean Code” but mostly basic DRY techniques. The product, itself, was plagued by being in a split-brain reality of Backbone and React (we were migrating as much as possible to React). We never addressed performance low-hanging fruit for large projects and it’s because it affected relatively few customers.

That said, pairing has its ups and downs. I think as other folks have said, you miss out on being able to go deep on problems that require ruminating. You also lack overarching reviews that help to keep architecture clean. The code might be clean, but the architecture might get really messy really fast.