I understand that. I've been thinking about that for some time, in order to have a good and all-rounded exercise program. For example, headstand or scorpion could be very difficult, but they certainly don't give some of the benefits cardio exercises give. So I'm not saying that "difficult" asanas can replace cardio-workout.
But does it means that going fast is necessary for the 'cardio-pulmonary load'? I am still conservative. Based on my experience, the variations of sun salutation could be challenging not only in the way that the poses are difficult to maintain, but also makes my heart beat crazily, not slower than jogging. So it's very "cardio" in its literal sense. Certainly the breath, though it wants to go fast (sometimes the urge is so large that I have to add one extra breath), are controlled to sychronized the movements, and depth are emphazied together with ujjayi. So I'm not sure whether such "breath-controlled" cardio can cover the benefits of cardio. I didn't notice any scientific investigation on that, by the way. What do you think?
Also, I don't mean to make it really slow. It's about 3-6 seconds per flow.
It's interesting to investigate further on that. For example, it's easy to understand what it means by exercising the heart, and the Ashtanga sun salutation certainly does that. But what precisely is "exercising the lung"? The lung is itself a passive organ, controlled by other muscles. Do we mean:
1. exercising those muscles for inhale/exhale;
AND/OR
2. Promoting higher rate of gaseous exchange in the lung?
Probably both. I don't mean to simplify matter, since there may be other means, beyond our current state of knowledge, that cardio-exercises are UNIQUELY beneficial to our body. But at least I think hard version of sun salutation can give 2 above, and enhanced by adding ujjayi.(and possibily 1 too, though it's in a more controlled and slow way, instead of pumping quickily) For 1, I use kapalabahti/Bastrika 110times/round, and 3 rounds both in the morning and afternoon. Such pressure also enhance gaseous exchange.