True, but the overhead is large. I graded into linear algebra and intro calculus, so there were a lot of students - I think 150 or so - and most of them were wrong.
Graders know that wrong homework takes much longer than correct homework to grade. It's correct? Full marks, move on. Is it wrong? Well, how wrong is it? Did they make a bad assumption, but followed it through to its conclusion? Did they forget a minus sign? Or is it complete hogwash?
So it might not be 10 hours, but still would be around 8 hours. And that's still too much.
When I was grading homework, it took about 5 hours a week per class per run through. They didn't pay me enough to make sense for it to be 10 hours.