"Willow always had all the power. Even before she knew what to do with it. It came so easy for her. The rest of us, we had to work twice as hard to be half as good!"
― Amy Madison, Buffy the Vampire Slayer
"When was it that the transformation to the legendary warrior of the Saiyan race was reduced to a child's plaything?!"
― Vegeta, Dragon Ball Z
"It's strange. For the longest time, I've been watching this boy. Just as Ri Boku-sama did. Observing him mature...His growth but...his rate of growth has been...Far greater than anything we predicted. Just when did he reach this level of strength?!"
User can improve their natural abilities far faster and to greater extent or beyond the limitations of other members of their species. This applies at a genetic level, allowing the body to keep up with the users 'own rapidly developing skills/physical attributes, and at an intellectual level, to which their mind/mental capabilities can develop beyond their normal limits. Users with superpowers benefit the same way, nd even allows them to awaken any latent abilities within them.
As a result, user can (eventually) benefit from training that would otherwise cripple or kill a normal human being.
James "Bucky" Barnes/Winter Soldier (Marvel Comics)
Finesse/Jeanne Foucault (Marvel Comics)
Xiao Yan (Dou Po Cang Qion)
Students at Shrek Academy (Douluo Dalu)
Kylie Galen (Shadow Falls)
The Young Guardians (The Young Guardians)
Willow Rosenberg (Buffy the Vampire Slayer)
Prue Halliwell (Charmed)
River Tam (Firefly)
Chance Harbor Circle (The Secret Circle)
Cecil Harvey (Final Fantasy IV); after becoming a Paladin
Minato and Minako/Hamuko Arisato (Persona 3)
Yu Narukami (Persona 4)
Jake Muller (Resident Evil 6)
Alex Mercer (Prototype)
Galen Marek/Starkiller (Star Wars: The Force Unleashed)
Milla Maxwell (Tales of Xillia)
Alice Fuji (Arachnid) was able to become an incredibly dangerous assassin on the same level as her mentor after only two weeks of training.
Ichigo Kurosaki (Bleach) grasps the basic control of advanced techniques of Bankai in 3 days where it would take several years for even the most gifted Shinigami.
Son Goten and Trunks (Dragon Ball Z) unlocked the base Super Saiyan form with ease at age 7 and 8, respectively.
Despite his childish intellect at best, Majin Buu (Dragon Ball series) is surprisingly able to learn new techniques and tactics on the spot, even after only seeing them performed once.
Caulifla (Dragon Ball Super) is a female Saiyan from Universe 6 said to have far more potential than her older brother, an experienced warrior...
...became Super Saiyan after being instructed once...
...reached Super Saiyan 2 afterward...
...and further developed by battling against Goku in the Tournament of Power.
Medaka Kurokami (Medaka Box) is naturally gifted, able to learn, memorize, recall, understand, master, and utilize anything and any skill by experiencing or observing it, within seconds, and has a limitless capacity for learning and growth.
Having an incredible gift for Haki, Monkey D. Luffy (One Piece) was able to learn everything Silvers Rayleigh had to teach him in a year and a half despite the old veteran saying that two years was too short a time to learn the basics.
With just 2 months of training, Tokitou Muichirou (Demon Slayer: Kimetsu no Yaiba) was able to become a Hashira/Pillar, one of the strongest warriors within the Demon Slayer Corps' ranks.
Whether it be martial arts or his military career, Ri Shin (Kingdom) possess adaptability and development that can only be disturbed as monstrous...
...by the time he was 14 years old, he quickly became a master swordsman with martial aptitude that suppressed the normal limits of an average soldier of the warrior caste,...
...and in the span of 9 years, he has risen through the ranks of the Qin military with incredible ability and talent as he became a 100-Man Commander...
...and later a 300-man commander in his early teens.
Already an impressive 1,000-men commander, Kou Yoku of the Thunder (Kingdom) was able to lead a hastily assembled 5,000 strong army just a day after being promoted.
Aang (Avatar: The Last Airbender) has incredible aptitude in the bending arts, becoming an Airbending master at age 12 and a fully realized Avatar within a year.
Michael Holt/Mr. Terrific (DC Comics) has a "natural aptitude for having natural aptitudes," allowing him to easily pick up and retaining complex skills and abilities that others spend their entire lives perfecting.
As a teen, James "Bucky" Barnes (Marvel Comics) studied every combat move of Steve Rogers in a few days and was trained with the same instructors who taught him.
Willow Rosenberg (Buffy the Vampire Slayer) has a natural talent in magic and witchcraft, becoming so powerful after a mere four years of practicing it that, according to Amy Madison, other magic practitioners had to "work twice as hard to be half as good."
Jake Muller (Resident Evil 6) picks up skills very quickly, learning advanced combat skills quickly enough to defeat several more experienced attackers and learning how to speak and read fluent Chinese after a few months of listening to it.
Milla Maxwell (Tales of Xillia) has an impressive growth rate, becoming proficient in swordplay after only a few lessons and learning how to perform Spirit Artes instinctively.