====== I'm using Windows 10 - why does it take so long to switch to and out of the game? ======

The Windows 10 April 2018 Update introduced a new feature which leads to slow tabbing, starting and closing of Settlers 3 and other Fullscreen games. To fix this issue navigate to:

Windows Settings -> System -> Focus Assist -> switch "When I’m playing games" to off 

//Thx to Incinerate für the hint//