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[Eagle of Fire]

The logic behind this is if you play a game in Windowed mode, the computer need to recalculate everything (graphical wise) depending of the size of the window instead of having it directly set as "default" in fullscreen. At least that's how it work for most old games, and this can be a very tedious task for your CPU (thus eating ressources) unless you have a good graphic card installed on your system.