We don't have to assume anything; the text tells us exactly why she's so much faster:
So how fast is that? The text tells us that as well:Xiao Longnü was taught how to separate her mind for two uses by Zhou Botong, the Left/Right Mutual Combat Technique; between her innate abilities and his teaching, her martial arts had multiplied. With Yang Guo forming the Dueling Sword Combination and using the ‘Pure Heart of the Jade Maiden Sword Technique’, few have matched it under the heavens. Now, as one person, she can use two swords simultaneously, with outstanding power. Regardless of how two people’s intentions are interlinked with each other, it’s still inferior to one person’s lightning-like alertness during battle. At this moment, although her sword method’s energy and strength weren’t as good as two people teaming up, her moves are faster in comparison by many times.
Yeah, opponents as skillful as Yin Kexi can't even see her attack.Yin Kexi was deeply surprised, he thought, “This sword may not have hit me, but I was powerless to guard Zhao Zhijing, that is also a disgrace”. His opponent’s move was very fast, he just couldn’t follow her twin sword’s route and oncoming force; if he continued to fight her in this way he will definitely lose. The more he thought of this, the more cowardly he became; with a swing of the Golden Dragon Whip, he bellowed: “Miss Dragon Girl, please be merciful!” Xiao Longnü didn’t care since she didn’t regard him as an enemy, or a friend; without slowing her pace, she stepped twice to the left. Yin Kexi followed with one turn, still wanting to protect Zhao Zhijing, but suddenly heard moaning behind him. Slowly turning his head around, he saw Zhao Zhijing’s robe sleeves had been neatly split into two parts by the point of her sword and blood came running from the wounds. How Xiao Longnü had pierced him, the other people were still completely baffled by it. Her rapid and skilful sword methods had reached a point where not only do her routes leave no trace, it’s as if she can block one person and still injure her foe at will.
There's no one in the Mongol trilogy who fights as fast as Xiaolongnu does: not Guo Jing, and not Zhou Botong. In fact, the only characters in Jin Yong's writings with comparable speed are Dongfang Bubai and Ah Qing.
In this case, we're left with the result that the speed of a + b is much slower than x + y. The conclusion shouldn't be that this is impossible, but that there's something special about x and y that make them so much faster.
9-Yin doesn't defeat the Jade Maiden martial arts. What happened was that Wang Chongyang took some ideas from it and used them as a counter to the Jade Maiden Heart Sutra. There's two problems here: the first is that even if you're as smart as Wang Chongyang, he had to spend some time figuring out the counter. The second is that Jade Maiden Swordplay is a totally different animal, and there's no way to know if there even is a counter for it.
They were worried that she would fall off the cliff so they tried to help her out. It was obvious that she had the upper hand the whole time.
And if a skilled and experienced combatant can figure out that her opponent is stronger than she is after just a few attacks, then you'd think that she might have some idea where they stand.