Apologies for the slow response. It isn't the hard or the easy parts, to me. Seems it is more the repetitive versus quickly done parts. A long climb up a hill? I'll be chatting in my head rather quickly. A series of up and down? A hill that varies? I could see those keeping me occupied.
I can still lose my train of thought while mid maximum effort deadlift, or pinned on a chunky downhill MTB section. I think it's less about the effort and more about the familiarity. I tend to have a clear mind when I am taking in new information, like during a ride in new place.
I used to cycle to work in the pre-2020 era, but during the first few months I remember how everything in my mind was about the bicycle and the road and forcing myself not to stop. After this first period the body adapted and the usual train of thoughts returned. When I noticed I was thinking of mundane things during the ride was when I considered that my body had adapted