One of the most annoying peculiarities about SCL is that, if you run in fullscreen mode and you alt-tab back into windows your OS is almost useless - the game appears to hog all CPU cycles. It's a weird bug that I can't remember seeing in any other game.
If you turn off full-screen mode, you can alt-tab normally but you'll have that annoying top window border... but you can also run it in a fullscreen borderless window and get the best of both worlds.
What you need to do is on the Steam Library, Right click on SCL and then click on "Properties." In the "General" tab, click on "Set Launch Options..." and then enter
without the quotes. That will give you a borderless window that looks and plays just like full screen
(Credit for this goes to Chojey from the SCL forums.)
“Microtransactions? In a single player role-playing game? Are you nuts?”
― CD PROJEKT RED