It’s extremely efficient given the low calorie density, high fibre diet and extremely limited grazing time (dawn and dusk) of crepuscular rabbits.

Think about ruminants for comparison. They spend all day every day grazing on the same kinds of foods as rabbits. Rabbits have a much more rapid metabolism (faster resting heart rate and ridiculous athletic ability) than, say cattle, yet they manage to extract more energy in less time eating. Rabbits are a marvel of efficiency!