I'm not sure about arthritis but I have a 10 mile cycling knee as the result of a basketball injury about 18 years ago. When cycling my knee starts to complain at 10 miles every time, if I go 20 I'll be limping the next day, and the last time I did 40 I had to be helped to the car.
Yes, my bike is properly fitted.
I finally realized that what irritates it is high torque, not the revolutions. It's hilly enough around here that high pressure on the pedals is unavoidable, even in granny low gear, so I recently added an electric assist kit to my bike. I press the magic button on the steep hills and the pressure I have to apply to the pedals via is cut in half, and my knee is much, much happier for it.