Fast Dormancy, in simple terms, is an Android trick to reduce battery use during phone and data exchanges via the phone's radio(s). There are some carriers, notably Rogers in Canada and Bahamas Telephone, who do not support fast dormancy. Trying to use fast dormancy with a carrier who doesn't support it results in the phone freezing up over a span of a few hours. It has the feel of a memory leak freezing a device. Some phones (Samsung and DROID RAZR MAXX are the two I know about) can be tweaked to not use fast dormancy. Rooting is required; Sammy uses simple type-ins, the MAXX build.prop file has to be edited (three lines of parameters added).
What's the situation with the One M8?