I came across an interesting article on written feedback in The Teaching Professor (found on the TLC's Resource page, scroll to the bottom click on the link and log in using your UCIT information).  This one-pager summarizes a couple of recent studies; the results make a lot of sense:

• The overwhelming majority of students read the written comments they receive from instructors on their assignments.
• Students tend to prefer rubrics over paragraphs. They like the visual display and feel that rubrics assist instructors in keeping assessment fair and consistent.
• Vague comments aren't helpful; students need focused feedback so they know what they're doing right and what needs improvement. Comments need to provide guidance.
• Feedback should relate to the assessment criteria. If not, students get confused as to how they're being graded.

The central message I took from this article is that it isn't a waste of time to provide students with written feedback, but a method that instructors can use to facilitate learning, individualized to the student. The article stops short of discussing strategies for the busy instructor, such as how to provide written feedback with large class sizes. It is helpful to know what is effective, but instructors also need pragmatic strategies for implementing good practice in the classroom they have.

Magna Publications. (2009). Written feedback: What's most
and least helpful. The Teaching Professor, 23(2), February, 2009, p.4. Retrieved
March 2, 2009 from https://www.ucalgary.ca/~commons/tp/0902TP.pdf.