Responding to students' writing involves far more than simply marking errors in punctuation and mechanics. Most grading time, by far, is devoted to commenting on focus, development and arrangement of ideas, the quality of arguments, and other larger issues.
Tell students in advance specifically what your expectations are for high-level writing skills. Then focus your commenting on how well students meet those specific criteria. Also consider developing a checksheet or some other commenting guide to help you comment quickly but thoroughly on the points you decide are most important for a given assignment. (See the samples in "What makes a good writing assignment?" and "How can I handle responding to drafts?")
