At what point does playing 14+ become less tiring?

Right now around 16.3k, I would expect myself to have adapted already, but I still get completely worn out after two hours of 14.6-8. Especially since upward from .7 are mostly stamina heavy charts.

A lot of rankers, or even close to max rating, play these charts effortlessly, I’ve even seen some wearing long sleeves (or even sweaters) while playing!

In people’s experience, at what point does it get less tiring? Before I get there, how can I physically prepare pre-session and post-session to maximize my improvement?