I think perhaps expectations on teachers have changed in the past 20 years. I went to a pretty bad school, but my majority of my teachers did not work hard, did not do marking, did not plan lessons.
Maths - we'd work our way individually through a textbookat our own pace. Every few lessons the teacher would demonstrate something from the book to the class, otherwise, you just read the book, and if stuck asked the teacher for an explanation. Books were checked every couple of months (hed mark in lesson when we were doing our books)
There was more planning insome subjects - science, geography stand out, but even then it was mostly just working our way through books. one maths class did crosstitch rather than lessons!!
History- blackadder was out on, we could watch it if we wanted, or equally quietly chat.
English - usually reading through a book as a class or an ongoing project. I used to nap in English lessons (Gcse year)
French - would just do excercises from a textbook and swap with classmates to mark, so the teacher didn't have to.
I'd assume things have changed a lot since then (I hope they have), but teachers that I know go out every single evening, go out weekends etc work less than a 40 hour week, and still do well at their job.
Like any job, it can expand to fill the time available, but that isn't necessarily better.