Given that the earliest the thread could check that its work isn't needed is after the image is 'rendered', there's not much more work that it does (the remaining tasks are not generally time-consuming) before it gets a chance to work on another image.
Hmm, I guess the rendering is a monolithic process that really cant be interrupted then. That's really a pity since the delay is really painful. Depending on the images (on the discarded contact sheet), the delay could sometimes be > 10 seconds.
Anyway, from what you are describing I don't think we can expect this to be solved soon (if at all). I'm therefore closing the issue.