With a proper fit and smooth pedal stroke, the upper body shouldn't be moving much at 5/10ths. In fact it really shouldn't be moving even at TT levels of effort. All that movement is wasting energy. Watch a pro race and you'll see smooth riding even at race speeds until they hit the sprints/climbs.
Saddle too high is a common cause of this, since that forces the rider's hips to rock to reach the bottom of the pedal stroke (and then the whole torso shifts around to compensate). However, some riders just never learn to be smooth. For them, some time spent riding rollers can result in surprising improvements in their speed and endurance.