Grand action has improved control, repetition and speed as its hammers move vertically on their own weight and whippens have double repetition levels.
Pianos are like cars: They are made of two systems, the musical instrument and the action, like the car and the engine.
A grand piano has always more “efficient” action than an upright piano due to its inherent design and architecture. It is a piano for advanced level, performance and recordings. The choice of professionals who will always prefer a better action than a better sound if they cannot have them both.