Glaciers = crevasses, which you either have to go around, climb over or cross on snow bridges, if they exist and aren't too rotten. The two first methods are VERY time consuming. Crossing on snow bridges is not particular fast either - you have to make reasonably sure that the bridge won't collapse (which takes time), and skiing with proper use of safety rope isn't exactly done in sprinting tempo. It is a slow, steady trot, with constant attention from anyone in the team. The idea is that once one person falls through all the others will throw themselves to the snow and dig in with all they have. This works as long as the rope is reasonably taut and everyone is attentive.