"I think [Harden] is very underrated as a defensive player," Rockets head coach Mike D'Antoni said postgame. "He's always been one of the best post defenders in the league. ...You can't ask for a whole lot more." https://www.si.com/nba/rockets/features/james-harden-key-rockets-small-ball-defense
I believe his perimeter defense should improve with his weight lose and agility focused training but it will never be as efficient as his post defense. Harden is built for power and deceleration.
Think it takes more energy and focus to frequently venture out to the 3 pt line.... Folks are also shooting way behind the arc now, so do the Rockets.