Blender render very slow - use 40% cpu only 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.

NOTE:

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.
This isn't really a problem with Blender that could be fixed, most likely it's just that the computer, after being used for a while, heats up. It also depends on how many things you are running at the same time.