I have not found a setting within the Steam mode to change input speed. I did boot into the desktop and slow down the mouse cursor there. But it does not seem to carry over into game mode.
The game I am playing does not have a cursor speed setting either.
I know for trackpad/joystick/gyro mouse you can definitely adjust the sensitivity, but I’m not sure about adjusting sensitivity for an actual mouse (if that’s what you’re talking about).