When I first started teaching "English Teaching Methods" courses, I used Rhoda Maxwell and Mary Meiser's textbook: Teaching English in Middle and Secondary Schools. They described an approach to grading that made sense to me--and that has been reassuring to the many students and teachers I've talked with over the years. Basically, they divide grading into three levels, labeled simply "Level One," "Level Two," and "Level Three." Here's my memory of what they say.
Level One: This includes writing assignments like brainstorming, freewriting, journal writing, etc. Since this type of writing is intended to help students explore ideas, the feedback from teachers can be minimal. In fact, usually it's enough for teachers to just check to see if it's done without giving any feedback at all.
Level Two: Writing that is a little more formal: drafts, homework, essay writing in exams. With this type of writing, teachers can read relatively quickly in order to check content. Feedback could address more global issues rather than the specifics. This type of writing isn't about correctness so grammar and punctuation shouldn't receive our attention (unless it's in the form of identifying class patterns of errors/problems that could be addressed in instruction rather than in writing).
Level Three: Formal writing assignments such as final drafts of essays. It's really only this type of writing that teachers need to respond to in more depth. Students have had the chance to explore ideas, perfect organization, and revise/edit. This is where we give more thorough response.
There are exceptions to this outline--but the point is that teachers shouldn't give exactly the same kind of attention to every piece of writing. If we did, we'd soon stop asking students to write because the paper load would be so heavy. We need to consider what the purpose of the writing assignment is, whether students need the freedom to explore without worrying what their teachers think or how grammatically correct their writing is, and whether students have had time and support in creating their best piece of writing.