Korra is the first Avatar known to metalbend. While Toph invented the subset of earthbending, Aang always had trouble with it, even expressing during one adventure that he wished he was able to do it. Earth is diametrically opposed to Aang's natural ability in air, so he struggled with any
ability that related to earthbending. Even in the flashbacks to his later years, Aang is never shown metalbending, indicating he was not proficient even as a fully realized adult Avatar. Korra had no such problems, having already mastered earth before she started learning metalbending.
Korra was able to hold her own against the advanced metalbender Kuvira in the fourth season, and she displayed an ability to effortlessly blend metalbending into her attacks and defense.