This clip features a common lat sao sparring session that we practice at our school; it was filmed towards the end of their 3-hr practice.
Wing Chun Lat Sao and Chi Sao training is not like normal sparring or real fighting. As you may notice, sometimes they pause before attacking, that is because they may be waiting for the other person to attack so that they can practice blocking and countering. Of course, there are some minor errors and habits that needs correction in this video (mainly because of fatigue), but that is why this kind of practice exist, to correct those errors. You should also take into account that they are actively trying to attack and defend each other.