If you switch the ECO button off the car will accelerate slightly faster (normally). If using Cruise control it will cause the increase in speed by the button to be slower. For top performance for a bit to see the difference, find a suitable section of ‘road’, switch off the ECO, and put Gear stick into ‘S’. Then try out the throttle now. Then compare again with in in the normal ‘D’ and ECO back on an throttle it again.