Here's a video capture showing the issue:

https://vimeo.com/229348566
password: RB.CYCLES

Task manager graph shows CPU utilization not speed. It is logical that there's less computation while fetching transparent tiles, hence the lower utilization. Also the model seems to have relatively simple shader, hence not a big difference in render speed between geometry and background.

No. That's not the point. Watch my recoreded video and you will see that the speed of the tiles in the transparent areas slow-down extremely. When moving the window they start to render in an expected and correct speed. It's not correct that the tiles will take ages in the transparent area. The same issue you'll have randomly in the areas with geometry. When moving the window the rendering speeds up to the maximum speed. When minimizing the window the geometry area seems be rendered efficiently till it comes to the transparent area. The slow-downs and sometimes freezes of tiles is not normal.

There is likely some kind of bottleneck in drawing huge resolution images, and moving the render windows stops it from redrawing. Such a bottleneck will have a bigger impact in transparent tiles that render quickly.

You could try changing User Preferences > System > Images Draw Method. Also rendering without a render window will probably be fast.

Hi Brecht,

thanks for the hint. I will try this and come back. Yes, I assume that this effect in my video is caused by the extreme high resolution of the image.

But I have the impression it's not only a redraw issue. It really slows down as long as I don't move the window.

It's because Cycles can be blocked by the drawing code when handing Blender the tile data.

OK. What I do now . I start the render and zoom in. When I zoom out the tiles slow down and the estimated render time is over 12 hours. When I zoom in at a maximum the tiles speed up and the estimated render time is now 5 hours. Looks like this is a fair workaround. :-)

Is this problem still seen in the latest builds?
@Sergey Sharybin (sergey), are you still dealing with it?

Mark it as a todo.