The bot is delegating what it presents to the human, though. So just present the human with images that have already faded in (and if no image has finished fading in, switch to the images from the other captcha).

Edit: Unless, it occurs to me now, Google is monitoring how the user responds to the fade in. When in the fade process do they click the square, for instance.

Still seems like it wouldn't be all that helpful, though.