The vast majority of people who scale Rails sites (as far as I can tell) do so by breaking their apps into services. ActiveRecord god-objects do not make that step in an app's life cycle easy, and separating persistence from business models is likely to be your first important step when refactoring for scalability. Fragmenting a User model bloated beyond all hope of sanity is almost a rite of passage at this point.
I often think the only Rails developer who hasn't found ActiveRecord bloat to be an irritation and an obstacle to scaling is DHH, and although that makes his opinion on the subject even more valuable than it would normally be, he doesn't talk about it enough in my opinion. Pretty much everyone else, as far as I can tell, responds to Rails scaling issues by creating services in Sinatra and/or divorcing business modeling from persistence logic.
I want someone to challenge my prejudicial assumption here, most of all because I appear to be saying "Rails can't scale," which is BS, but also because I'm very tempted not to take this discussion seriously at all if it doesn't address this point. It's just the crucial point in my opinion. I like the whole OOP thing but to me the decision to use something like Objectify is all about scalability. That doesn't just mean performance; it also means retaining readability when you have a lot of code. Single Responsibility Principle makes code readable.