Many modern cars come with an “Eco” mode designed to optimize fuel economy. However, tests have shown that Eco mode can be disappointing in both city and highway driving. The reduced throttle response in Eco mode can encourage a more gentle driving style conducive to saving fuel, but the need to still achieve the speed limit within the time-bound test meant testers had to push the vehicle a bit harder to essentially overcome the hesitation added by Eco mode
Source: Consumer Reports