Hey there, sorry to hear about your situation. Have you tried any of this?:

First, make sure your computer is running the game using the correct graphics card. Depending on your setup, the game might run using the default integrated card, which could result in slowdown. Make sure the game is running with your prefered graphics card.

In general, make sure everything is updated. DirectX, .NET framework, drivers for audio, video and controller drivers.

If you have a controller plugged in, try unplugging it and running the game and see if the issue is fixed.

Slowdown can also be caused by background programs interfering. It's hard to point out exactly what could be causing it, so I suggest running the game with as few programs open as possible and see if that helps.

Other potential solutions:
-Disabling display scaling on high DPI on the game's .exe file (Properties -> Compatibility -> Disable display scaling on high DPI).

-Try disabling the Steam Overlay and Steam's FPS counter.

-Running the game in compatibility mode (Win XP or 7)

-Restarting your computer

Hi! Got mine fixed running it on admin and win 7 compatibility mode! Thanks!