I have a 4K monitor and before I have logged in, the mouse moves really slowly on the user login screen. Once logged in, I have configured the Tracking speed in the Mouse System Preferences to move fast. Since there is no known way to configure the default Tracking Speed before I've logged in, is there a way to select a login user without clicking on the user and using the keyboard to select a login?
You should be able to choose any of the login users either with an arbitrary arrow key (if the user name is somewhere in the middle more than one hit is necessary) & enter or by entering the first letter of the user name (or the first two if you have several names starting with the same letter) & enter.
Then enter the password.