Cycles GPU rendering is super slow with bigger image size For some reason Cycles GPU rendering speed increases enormously when using bigger image sizes.

I am now doing 6000x3000px image and it takes almost three second to calculate one sample. And it is not the case all the time. When using smaller images (like 1920x1080px) rendering is as fast as expected. Also, sometimes rendering starts quite fast, but then slows down to one sample at second speed. 

And I know that the doubling of pixels means four times more calculation. The Increase is now more than ten times. I might be from 5 minutes to 8 hours. 

I am using 640x640 tile size but I have experienced with different tile sizes too. Using bigger tile size than that is not possible because blender crashes (which is odd thing too of course).

I use the newest build of Blender 2.8 but I have experienced same issue with 2.7. My first thought has been that it is because of the memory issue, but why image size would have effect on that? Blender is able to download whole scene to GPU memory and memory peak is not more than maximum of 5 gigabytes.

I made a thread about same issue 7 moths ago, but at that time I wasn't up to date about the problem: Cycles GPU rendering turns to really slow with big image sizes.

Someone had the same kind of issue here (https://blenderartists.org/t/cycles-gpu-render-speed-drops-extremely-when-increasing-render-size/638059), but the solution of closing T-panel in the UV/Image Editor sounds really odd and didn´t work in my case.

Does anyone one what might be the case? 

Specs: Windows 10, Inter Core i5-7600 @3.5GHz, 16 GB RAM, NVIDIA GeForce GTX 1070
You have enough capacity on your tech specs to accelerate the render.
I wouldn´t trust the speed of 2.8 just yet. Instead let´s focus on blender 2.79. Since you don´t make any mention what is exactly what you´re rendering I´d assume it´s a lot of hair/grass or either transparency with lots of bounces.

Bounces on the WORLD are one of those things that can hog up your memory pretty quick. Reduce the bounces on the world to 12 (first). Also on Render settings: Samples render: 24. Lightpath: Bounces: 4. Transmission: 1 if you´re not using any crystal surface you don´t need this). Uncheck reflective caustics and refractive caustics.

Make your 4k image render size: 3840x2160 pixels.
Performance tab (render): uncheck use Hair BVH and set tiles to HILBERT SPIRAL and set the tile size 32x32 (I know, crazy -right?)

AND THIS IS WHERE THE MAGIC HAPPENS:

Create a unique material on the scene assign it to a sphere.

Select SCENE TAB on the properties, and use the material as a global override on the scene (we do this so you don´t have to switch on and off every material on your scene). And hit render.

With a simple diffuse material and your tech specs, your 4K render should be ready in about 2 or 3 minutes (maybe less).

If this is your case, you now know that the slugness is coming from a material rather than the scene setup (of course you can up raise your sample settings all of that) but now you know where to debug.

On the contrary, if your render delayed more than 10 minutes (with material override) it means the polycount on your scene is too large and the render is using disc space as temporal storage data when calculating, hence "fill" and "deploy" until next tile is calculated.

If you make these test and still do not get any variant results, please edit your questions and write more details about what exactly are you doing with the materials and the models.