[adamesque]

The parent hints at this by talking about minimizing unpredictability and describing Facebook as "people oriented" - it ultimately boils down to user experience.

Think of it this way: if you know a certain function will reliably take a little while to complete, you can justify the effort of adding progress indicators and other feedback to let the user know.

But if query performance is unpredictable, even planning the UI design becomes difficult - not to mention the end user's experience.