WD Green is for what I call "secondary storage": it's data you want to have available "when you need it", but you don't need it very often. For example, someone who takes a lot of digital photos & wants to store them, but maybe accesses them once a month (or less), would find a WD Green drive ideal. It draws less power, & tends to also go into a "standby" mode when it's not in use. Hence the "Green" designation.
If you installed your OS on a WD Green, or installed apps that you regularly access... then the best way to improve your performance is to migrate to a WD Blue or WD Black drive. They run faster (7,200 RPM vs. 5,400 RPM) & are meant for day-to-day usage (the Black being heavier-duty with a longer warranty).