Twenty letters never fit in one free period.
Every fall the requests stack up, and every student deserves a letter that could not have been written about anyone else. But after the fifth one, even a caring recommender starts reaching for "hardworking" and "a pleasure to have in class", the phrases admissions readers have learned to skim past.
The hard part was never the typing. It is pulling the right moments out of a year of teaching and getting them onto the page before the deadline.