The blank page is the expensive part
Educators know exactly what they saw and why it mattered. What costs time is starting from nothing, hours later, when the specifics have blurred. The moment a child changed their mind, tried a different approach, or said something in their own phrasing is what makes a story worth keeping, and it is the first thing memory loses. A rough note captured at the time removes most of the cost, because the recall work is already done.