The ISM Road Speed Governor is used to control how fast the vehicle may travel on level ground. The feature is always active. Depending on the setup, it can have an impact on how your vehicle will perform on uphill and downhill grades.
The Road Speed Governor is designed to LIMIT your maximum vehicle speed. If your engine achieves a certain speed and feels like it should have more power to accelerate, you are most likely traveling at the vehicle's governed maximum speed limit. Check the Accelerator Maximum Vehicle Speed parameter to verify the setting.
The Road Speed and Cruise Control Governors are designed alike. They both have maximum vehicle speed and droop parameters. If these parameters are setup differently, the vehicle will perform differently while using cruise. If the vehicle performs better with cruise off, the cruise speed settings and/or droops may be setup to take advantage of fuel economy and speed control.