Yup! I've been saying for years fix gears are 25-33% harder than freewheels. (Meaning the freewheel mechanism. present in both "freewheels" and cassettes.) I rode mine yesterday and am feeling it today. (And I went for a walk yesterday evening because I knew if I didn't today wasn't going to be fun.)
My latest saying. "You ride gears to make your body fast. You ride fix gears to make your body hard."