When I mark student work, I tend to make end-comments more frequently than I do in-line or sidebar comments. What usually happens is that I develop a digital form which I keep open in a window alongside that in which I open the document I am evaluating, and as I read the latter, I make comments in the former. From those comments, I determine the assessment I will accord to the student document, and the score and my comments are appended to the end of the student document before I send it back to the student and enter the grade into my records. This allows me to evaluate papers fairly quickly while still allowing for reasonably thorough assessment.
It also allows me to set up yet more practice for my students. I have found that when I do leave in-line or sidebar comments, my students make no other adjustments to their papers than the things specifically commented-upon. When there are systemic issues, they are not corrected throughout the students' papers, and the students tend to repeat the same errors in future work. (Too, if I do miss one or two errors--as happens, since I am but human--they are not corrected when the students go back over their work if all the students do is "fix" what is marked.) Offering end-comments, however, denies the students the easy fix-what-is-marked-and-everything-else-is-fine option for correction. It requires students, if they will actually work to improve upon their papers (and several have told me that they do not), to read through their work carefully, paying attention not only to the surface-level concerns that they and many members of the general public equate with "good" writing, but to the deeper issues of content and style that actually make for good writing. Admittedly, it involves more work for students, prompting complaints (oddly frequently from the same students who will beg for extra credit opportunities), but it also involves the kind of self-evaluation and assessment that typifies good writing, meaning that it is a valuable exercise for students to do.