There is a reason why my seat belt is fastened once I'm seated, and is only unfastened if I have to leave my seat (and that duration is as short as possible). They're not just for take off, landing and whenever somebody feels like it. And entering and leaving the jet stream can be a ride. Fortunately, coming back from NRT to SEA last Thursday was nice and smooth (and had a great tail wind for quite a while). SFO to KIX the previous week had some really slow ground speeds approaching Japan due to a nasty head wind. Less than 400 mph over the ground.