Thanks, Noel. My supposition is that Disable Fast Render is akin to doing a online render of a session in ProTools or Logic - - - that it is just a quickie pass through the file for convenience sake. Whereas if "fast" is disabled then the render is done with more precision even if it takes longer.
What you are describing is not intuitively gathered from it's name, but if you are right then that is a fantastic feature as I have spent 15 years waiting for BIAB files to render quicker after I make a change in the chord structure or hit the "Stop" button.