There definitely is something that bothers me about using fibers in order to hide an important optimization detail from the developer, and I agree it would be better to have explicit control.

However I see the core team's point that they don't want to completely overhaul the API in a way that's going to break arguably a majority of existing apps, and at the same require the developer to pay attention to more details than may be necessary.

However on the topic of breaking expectations about processing times, I couldn't disagree more. To me, the main benefit of MVC (or any architecture choice) is in isolating responsibilities, not isolating code execution. It's not a big mental leap to conceive of a ActiveRecord relation as declaring some data that's needed that may or may not be actually queried, depending on if the view needs it. This helps your MVC separation, because it means you don't need to extract view logic from your template just to make sure the controller doesn't load something unnecessarily. Lazy execution of this form is a very powerful optimization technique, just ask any Haskeller.