Parallel mode will refresh frequently for low depth, but agonisingly slowly for high depth. Because it does the same number of pixels in each, even thought latter case could require 256 times as much work per pixel.
It decides on the number of frames to draw, and draws total_pixels/num_frames each time.
We want it adjust num_frames at the start in a way that makes refreshing even. Two ways spring to mind:
* Look at the current depth, and make a guess. Of course, the depth doesn't tell you how much of the view will contain black pixels. The view could be 99% outside the set with pixels at much lower cost than the maximum.
* Try a few pixels, and measure how long it took to do them.
I favour the second approach. The thing is it's probably going to be hard to do without wasting those few pixels.