I'm going to disagree with this for science and engineering. All of my tenured colleagues work very hard. You have to if you want to keep running a lab, which is the main reason why people choose to become professors. If you stop working hard, you will stop getting funding and PhD students. The department will make you teach more and can make your life difficult if you are being a drag on the department, even if they can't fire you.
That said, for the liberal arts, history, etc., things may be different. They don't tend to have large labs and their goals are different. I honestly don't know.
Things are different for those doing theoretical science where the main instruments are pen and paper. External funding is often not required at all. This might not please the department, but the tenured professor would be just fine.
When I did my PhD I worked in Math and CS (plus had friends in Physics with similar observations): many professors on the theoretical side had no need for and were not interested in getting grants. They would get some minor travel money for conference expenses and pay themselves over summer so they would not have to teach then, but even if they got nothing they would be just fine on a base salary. And I am talking about reasonably well known full professors, not some young researcher at the end of a rope (career-wise).
Things are different when you have experimental labs to run. That is when you need external funding and have to do all that is involved in getting it (proposals unlimited, etc.)
That is definitely not true for many non-experimental science departments. Many students are TAs and as such care not a bit about grants. When I was getting my PhD I had to teach 4.5 hours/week (often structured as 6 and 3 at alternating semesters), which gave me my stipend and free tuition (and was a useful skill to polish). Maybe I spent another 4-5 hours preparing for classes and grading (we got student graders but I seldom asked them anything as grading was quick enough).
I was paid by the department and it did not matter to me whether my advisor got grants or not; what mattered were research interests and his guiding of research. This was the picture across the department; changing advisers was quick and based on mutual interest, not financials / grants.
That's a very light TA load. It's far more normal to have a 20-30 hour a week commitment to your TA responsibilities (multiple lectures, grading a few hundred assignments every few weeks, etc) so it's a lot harder to fit research around that.
Are you serious? Can you share the school and department? This is an honest question -- I always thought 4-5 lecture or classroom hours was normal. That's what I had and that's what my friends had. Most of us never even used graders assigned as grading duties were light and it would often be more trouble to explain how you wanted it graded anyhow.
I have never heard of a 20 hour TA load. Again, just wondering.
I was in a stats department for grad school. My assistantship load was nominally 20 hours, but I managed to teach a course as a GI in probably ~10 hours/week (doing my own grading). It took more time commitment if it was my first time teaching the course, and when I had to write/grade exams.
20 hours of work is pretty extreme and I suspect cuts down on your mental ability to do research (you have to take classes, too). I suspect the department usually has to say it is a 20 hr/wk position (probably so student gets free tuition), but the actual work is often less -- my friend who went to U of Maryland for PhD in CS had a similar experience to mine -- 7-10 hours a week of actual work at most.
This is just a couple of data points, though. Maybe we were just lucky.
That said, for the liberal arts, history, etc., things may be different. They don't tend to have large labs and their goals are different. I honestly don't know.