What I meant with the worn groove in the Dahon (Tektro 857AL) brake arm, is that during braking while the bike is moving, the spring digs into the groove and slowly makes it deeper. I suspect that when the spring gets stuck in the groove, it reduces the amount of spring force, so the movement speed of the arm is reduced. The groove on the right arm is much deeper than the left arm, so it's possible that when braking while moving, the pads are not engaging and disengaging at the same time, and the right arm is lingering longer on the rim.
When adjusting the centering of the brake pads while off the bike, the spring doesn't dig into the groove because I'm not applying as great of a force on the brake and the arms are not bending forward as when braking while riding. So even if I adjust the pads to center while I'm off the bike, when I am actually braking while riding the bike the pads may not be touching at the same time due to the spring getting pushed into the groove.
I've changed to the Shimano BR-T4000 brakes, which has an internal pivot, so it doesn't pivot on the boss and has less play in the direction of travel, and the springs run on the outside of the arms, instead of directly and behind the arms like on the Tektro.
The spring dug a groove into the back of the brake arm.