[nulld3v]

My understanding is that no smartwatch takes SpO2 readings during activity/workout tracking as the sensor is not accurate when you are in motion.

The 20h number I mentioned is from the OP: "For continuous outdoor workout tracking, Apple Watch Ultra 3 now gets 20 hours of battery life in Low Power Mode with full GPS and heart rate readings." Low Power mode disables AOD and SpO2 (which is already disabled anyways during workout tracking).

I doubt SpO2 is very battery intensive either, certainly it can't be worse than the heart rate sensor. Both sensors work in the same way fundamentally.