Cycles - rendered viewport really slow in Blender 2.8 So since months I am experiencing that cycles is getting slower and slower for me
in Blender 2.8 and I thought with the release coming it might become better.

However when I have a very simple scene and use cycles to have preview it takes several seconds just to update small position adjustments i make to a sun lamp for example. 

I do not know if I am having a setting wrong so it would be really nice if someone could help me.

I use a GTX 1070 as GPU and have enabled it.
Tile size is 240x256 (autotile)
viewport tile: I tried 64, 128, 256, 512.
CPU is i7 6700K and 16GB DDR4 2800Mhz.
Try to use combined GPU AND CPU for rendering. Using square tiles will increase render speed (any square numbers "X²" for better performance). You can find some sites to help you minimizing the render time.