I'm not sure this is always true. Perhaps some really experienced boxers also know how to mess with you technically enough to respond calmly without any need to rush anything. They are more relaxed. However those of us who are less experienced can not see as many opportunities and do not have as many tricks up our sleeve so we need to be quick to make advantage of the little we know in comparison. We spar competitively because we are always being challenged while the old masters can just play with their students and laugh!