issues with rendering

I noticed strange thing with rendering time. Rendering the same scene with the same settings takes a different time. I am using laptop and it is always plugged in and set to best performance while rendering but sometimes it looks like it do not use full recources of the computer. For example, if normally it takes about an hour to render a scene and blender works properly, you can feel that it is using a full power, because laptop temperature gets higher and computer is noisy (cooling system starts to work) and another time I restart blender the same scene could take 2-3 hours to render on the same settings, and laptop does not show any signs of higher performance. The same thing goes for GPU and CPU rendering. Maybe someone knows whats happening and could help me out on this?
Thank you in advance.