I'd also be interested to know what the speed of cognitive decline is OUTSDE the times that you are <birdwatching/hiking/whatever is claimed to cognitive decline>.

Like, if you have a baseline cognitive decline of 25% in 10 years, and you instead spend 2 of those 10 years (total time spent) watching birds to go from 25% to 20% reduction in cognitive decline, you haven't really slowed your cognitive decline, you've just effectively paused it during the time you were watching birds.