Marking essays can't possibly be a waste of time. How else is the student going to learn how to structure one?
By spending time in lessons looking at essays and discussing how these models are structured
By practising planning together - doesn't have to leave to an actual written essay, as plans indicate structure
By look at each other's plans and discussing them, then amending
By a teacher looking through work with a student and talking to them about it
By looking at exam mark schemes and picking out what is expected of them when writing an essay, learning how that might influence the development of an argument in a longer piece of writing
By ordering a set of pre-written paragraphs in a lesson to demonstrate understanding of how an argument develops
By improving an adequate piece of writing that is given to students by the teacher
...None of these things is the teacher marking an essay. All of them can support the development of essay-writing skills. All require students to demonstrate developing understanding. Looking at a marked piece of their own writing, unless done with elements of response, is a largely passive experience and as a measure of teacher effort-effectiveness, is not efficient