Hello, I think a problem with the mouse sensitivity or something, there's a kind of latency between the moment I move the mouse and the moment the game reacts, which makes the experience awful. I set the sensitivity to 0, disable mouse acceleration, I try with turning off the "Mouse smooth" option but nothing change. Is this how the game is supposed to be played? Or there's something to do? Thanks.

Edit: I specified Second Encouter, but I also have this problem on the first one.

While I cannot help, I can safely state it is not normal at all. The game is normally very reactive and it is necessary for the most pressing battles.

Have you tried turning off V-sync? V-sync can sometimes cause latency or unresponsive controls.

You can't fix mouse problems in this game... Only Serious Sam Revolution (only on steam...) has RAW input for mouse...

Have you tried Sharp turn in player settings?