As said by the dev on the forum here :

"To clarify, the game SHOULD go slow motion when you click in David's circle to shoot. If that's not happening, you're experiencing this issue." (and ennemies shouldn't accelerate like crazy when you shoot)
And
"This being my first game, I was unaware of the possibility that different refresh rates would affect the slow motion effect, and with Gamemaker, the engine/software I used to develop the game, there's not an easy way to go back and fix this. Actually, I have no idea how to even check for monitor refresh rate with the software. Only recently have I realized what is actually happening to cause the issue with the physics, and I suppose I should add some warning/disclaimer now to tell people about this. After all, these fancy 144hz monitors are only becoming more and more common, so the issues will show up more often."

As someone who played 95% of the game without any slow motion (and died way too much) yeah I think he should add some warning ^^
So I made this guide to spread awareness about this.

As said in the thread, to fix it you need to set the refresh rate of your monitor to 60hz (worked for me).

To change the refresh rate of your monitor (on Windows), follow these steps:

1/Right-click on the desktop: This will open the Display settings window.
2/Select “Display settings”: From the drop-down menu, choose “Display settings”.
3/Advanced display settings: Scroll down and click on the “Advanced display settings” link.
4/Monitor settings: In the advanced settings, select the monitor you want to adjust from the dropdown menu.
5/Refresh rate: Look for the “Refresh rate” or “Screen refresh rate” option and select the desired rate from the available options (e.g., 60Hz, 144Hz, 240Hz).
6/Apply changes: Click “Apply” to save the changes.

Additional Tips:

-If you have multiple monitors connected, make sure to select the correct monitor from the dropdown menu.
-For some monitors, you may need to adjust the refresh rate through the NVIDIA Control Panel (if you have an NVIDIA graphics card) or the monitor’s built-in settings menu.
-Some Windows 11 systems feature Dynamic Refresh Rate (DRR), which automatically adjusts the refresh rate based on your usage. You can disable or adjust this feature if needed.

And that's it !