rendering GPU load is decreasing Dont know where is problem, render image for 8 min, gpu max load, compute 100%, card temperature 70 degrees during render. And 5 min later same image, card compute 100%, but card temperature only 60 degrees, less power draw and render time 20 min... How is it possible?

edit: I found if I restart blender, first render take always 8 min. and if not restart every next render take 2x more time.. Its some bug?
It's possible that your card is decreasing the clockspeed because of high temperature.
You can use msi-afterburner (its free) to see all parameters of your gpu.
Maybe if you are rendering with gpu and cpu, the gpu doesnt render everything (stays cool) and cpu is slower (it takes longer).