The Speed Factor is multiplied by the Link Speed to obtain each driver's desired speed. By default, driver types 1-5 have a desired speed less than the Link Speed, and driver types 6-10 have a desired speed greater than the Link Speed. You can edit the Speed Factor for any or all driver types to calibrate travel times.
Remember that SimTraffic's Driver Parameters are applied globally throughout the network. The Link Speed may be a better setting to use if you are only needing to calibrate a specific section of a large network.
Also check out the other recent posts about calibration: