I know exactly what you mean. I ride on this stuff all the time. I finally ended up with Nokian 296 Extreme's, a bike with front suspension. And to go any distance. I had to get a really good seat post shock absorber. I run my tires at 22 psi front and 25 psi back. It still is a hard workout. But I do it by choice for a workout when I want to. I don't have to ride on it. I have ridden as far as about 15 miles over 3" bumps, but it took me all day and I was exhausted. whew!
This is winter riding!
The slower the better, yes. Patience is important. I’m talking first and second gear most of the time. I hope your knees feel better fast.