In one of our large Rails apps, we use a lot of inheritance. We don't like that fact, but that's how it is right now. So of course we have to keep preloading on with Mongoid. In the process of upgrading to Rails 3.1 and trying to use the new Asset Pipeline it is extremely slow. Here are couple examples:

Preloading ON: http://yfrog.com/kgmkfp
Preloading OFF: http://yfrog.com/nycmjep

I understand the need to preload models when using inheritance, but Durran mentioned that perhaps only the "inherited" models could be preloaded, which may help speed things up a little bit.