Your mileage will no doubt vary based on various factors, such as the PC you're using and the sites you're visiting. We'd add that browser power optimizations aren't completely new, for that matter. Some of Apple's Safari updates (such as the one in OS X Yosemite) have focused on improving battery life in normal use. Still, it's not often that you can simply flick a switch to get some extra surfing time. And let's face it: a little improvement in longevity could be crucial when you're finishing some school research... or just want to stay in the coffee shop for a bit longer.