I found the following issue with Blender render.
When I render an image after a fresh restart, it uses 100% CPU and finish in about 5-7 minutes.
But after a few renders, the same image will render in 43 minutes and use only 40% CPU
I think it is caused if I open more than 1 blender instance.
The only solution I found is a restart.
I have win 10.
After much research I found this is a win 10 issue, that is happening not only when running blender, I install MAC and the problem is solved.