[Crusader Kings 2]Very slow in fullscreen mode with upscaling

[Kzimir]

Hi,
I launch CK2 on my second screen with 1440p upscaling with this command :
gamescope -w 2560 -h 1440 -W 1280 -H 720 -- %command%

The game works and it's fully playable if i stay in window mode. But if you enable fullscreen mode with Win+F, the game is really slow when a new game is loaded and during a party.