Doubtful. It is a green drive and I know they have a variable RPM spindle speed. Maybe it's lowering the RPM to conserve power. When you check HDTune's health screen, does it show all properties as OK or passed? I know you said it was healthy, but didn't know if any of the attributes were out of spec, like reallocated sectors or pending sector count.
Other than that, not sure why it would behave that way.