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.
You should be able to do it with steam input
The only place I found anything about steam input was in the properties of the game and then under controller, I can set it to Steam input, but I do not see where I can make any changes to Steam input, and I am not using a controller but a keyboard and mouse.