Note: Doing the following will invalidate your track if you race online with the server running the original version of the track.
To lower the speeds you'll have to open up the track.ini inside the track folder. Look for the following lines:
track_asphalt_grip = 1.01 ; > 1.0 means more grip. This shouldn't have to change by more than +- 0.05
track_concrete_grip = 1.093 ; > means more grip. Again, keep between about 0.9 and 1.05
track_paint_grip = 1.22
Plugging in the values above should get you started. If you still need a tweak, it's best to play around with the values yourself to get a feel for how much of an affect a change has on the overall track speed.