Discounting Chan Ga Lok of BOOK & SWORD: GRATITUDE & REVENGE, who first appeared as a fully-developed martial arts master, Gwok Jing was the first major hero in Jin Yong's canon whose martial arts development the audience followed from humble beginnings to eventual greatness. At the end of LOCH, twenty-year old Gwok Jing was the world's seventh best fighter, but it was generally known that he was still only 60 - 70% as good as the Greats, and would not reach parity with them until he was in his thirties. That's a good, gradual procession that makes a certain amount of sense.
The next protagonist, Yeung Gor of ROCH, reached parity with the Greats at around age twenty...and with surprising abruptness. Only a good fighter when he lost his arm to Gwok Fu (probably considerably less than 70% of a Great), all it took was about a month of consuming snake organs and training with Dook Goo Kau Bai's Heavy Iron Sword to bring him to parity with the Greats, defeating two of them (the Golden Wheel Monk and Kau Cheen Yan) in short order.
Cheung Mo Gei of HSDS also reached Greats-level in his twenties, although unlike Yeung Gor's strange quantum leap, Cheung Mo Gei's advances came through an uninterrupted five-year period of constant training with the best internal energy method available in wulin at the time, and was tempered by the fact that despite his incredible power, Cheung never had the sharpest combat instincts.
Ling Wu Chung of SPW went from being a good fighter (like young Yeung Gor) to near-Elite level with the addition of Dook Goo 9 Swords to his repertoire.
Finally, we get to DGSD, where the quickness of advancement from zero to hero reached ridiculous new heights: Deun Yu went from a complete non-martial artist to being the world's third or fourth strongest practitioner of internal energy within just a few months. Hui Juk's case was even more jaw-dropping, going from extremely weak fighter to perhaps the second most powerful fighter in the Jin Yong canon after the Janitor Monk almost *instantaneously* after power transfers from the Siu Yiu Sect Elders.
It's almost as if Jin Yong got progressively impatient with letting his protagonists develop their skills at a believable, gradual rate and grew increasingly liberal about granting them elite-level martial arts sooner and with less fuss.