It has to do with how it updates the passes I believe. I have tried it on both machines that uses CUDA and on those it updates the seconds in "chunks" and seem to wait a bit for each pass. Here you can see that 100 samples took 59 seconds to render but only 18 seconds in Raytraced mode. (Both same size and I can not tell the quality apart)
The odd thing is that when the viewport that is rendered (cycles) is large (tested at 1200x1200px) then it pauses for each pass, but if I lower the viewport size to 400x400 then Cycles is almost as fast as Raytraced.
It refreshes the render window in slower passes and pauses between each pass. Don't know why. It seems to redraw the render view in lines while Rayraced updates the whole view in one go. How does it work for you?
I made a simple render scene, with a dark background and a transparent material so you see the redrawing easier.
Cycles render: 9m 23 sec
(Feel free to do what ever you want with this file) You can test it here:
Wineglass.3dm (372.7 KB)
Oh, by the way, I get worse banding in the shadows in Raytraced than Cycles.
The banding starts showing at 500 passes and becomes ugly at 1000. Here at 1500:
Here compared to Cycles: