Should prescale option be turned on? I tried with it off and sensitivity seems way too low.
Edit: Yes, it should be. Also, Mouselook speed should be set to twice that of the turning speed. From what I understand, mouselook is the vertical sensitivity while turning speed is the horizontal. By default, if both values are at 1, the horizontal sensitivity seems about half as much as the vertical, so setting Mouselook speed to 2 while turning speed is at 1 should fix it.