1. Download my attached project(782960 repro)
2. Build for Windows.
3. Launch the task manager and open the performance tab.
4. Run the game in full screen.
5. Let it run for 10 seconds.
6. Alt tab out.
7. Wait for 10 seconds
8 Alt tab back in, and repeat a couple of times.

Notice that CPU usage is increasing noticeably when project is running in background.
It is not as big on this project, especially on good machine, but with bigger projects cpu
increase was enough to set the fan going.

Screenshot of cpu usage: https://intra.unity3d.com/fogfiles2/78xxxx/782960_9biv49aogv140c1i/case_782960-Untitled.png