Well it defaults to 10.000000, which is full speed.
I'm using 2.440000 at the moment.
Did you try just going into Options>Character Mode and turning down the slider?
Anyway, here are my Character mode settings.

[Character Mode Options]
Mousewheel_Speed_Min=1.000000
Mousewheel_Speed_Max=10.000000
Mousewheel_Speed_Default=10.000000
Mousewheel_Speed_Current=2.440000
Mouse_Turn_Min=0.050000
Mouse_Turn_Max=3.000000
Mouse_Turn_Default=1.000000
Mouse_Turn_Current=0.168000
Edge_Turn_Min=3.000000
Edge_Turn_Max=10.000000
Edge_Turn_Default=5.000000
Edge_Turn_Current=3.210000
Camera_Lag_Min=1.000000
Camera_Lag_Max=30.000000
Camera_Lag_Default=5.000000
Camera_Lag_Current=7.090000
Fade Objects=1
Show Move Cursor=1
Mouseover Highlight Flags=74
CameraMode=0
LeftClickAction=1
Ceiling Draw Mode=0
PossessAsDefault=0
CameraFollowAlways=0
MouseMovement=1
Enable ScreenEdgeCameraTurn=1

Along with changing the current setting as olnorton mentioned you can change the minimum and maximum (Mouswheel_Speed_Min and Mousewheel_Speed_Max) so that the slider in the preferences screen covers a different range. I current play with min 0.5 and max 5.0 (half the defaults) and have sometimes used even lower. Be sure to make any changes in the sections of nwn2player.ini for character, strategic, and/or exploration mode depending on what you use.

On the whole the camera in NWN2 never feels just right to me--always seems too slow in some situations and too fast in others--but tweaking these settings can help.

Best wishes.