The problem is that all attacks depend on energy. The attack energy would ultimately have a formula something like:
E_impale = KE_Sayaka + KE_lengthening
E_impale = 0.5 * m_Sayaka * v_sayaka^2 + 0.5 * m_sword * v_lengthening^2
Where
E_impale = Total energy directed into The Stabbity
KE_Sayaka = Kinetic energy due to Sayaka physically moving to stab something
KE_lengthening = Kinetic energy due to the sword getting longer
m_Sayaka = mass of Sayaka in kilograms
v_Sayaka = Sayaka's speed at moment of impact in m/s
m_sword = mass of the sword in kilograms
v_lengthening = Speed of the tip of the sword as the sword length increases in m/s
We have assumed here that the sword's increase in length is only due to the change in length of the blade, measuring from the hilt. We can see that if the blade length doubles then the mass would double. We assume also that v_lengthening is constant, which it may not be, but that probably doesn't matter.
Ultimately, m_Sayaka will be something in the region of 45-50 kg, whereas the initial m_sword is probably only about 1.5 kg. The sword would have to double in length six times in order to approach that mass, while also having v_lengthening be some large number to have KE_lengthening be the same magnitude as KE_Sayaka. Meanwhile, KE_Sayaka already starts at a fairly large number even if you assume she's moving at only 5 m/s. 562.5 joules is fairly nontrivial, and 5 m/s is only about half the speed of Usain Bolt. Sayaka can absolutely get up to 100 mph (44.7 m/s) and has already demonstrated something resembling that speed in this battle.
Yes, technically, magic may allow Sayaka's sword to get longer at Ludicrous Speed and that's potentially a thing you could do. This is fair, but it's still clearly doing it just for the sake of doing it or having a Neat Trick in case of Absolute Fuckery. It makes more sense for now to focus on getting Sayaka's entire body up to something like 100 m/s or 200 m/s and rocketing into things at significant fractions of the speed of sound. She can probably take the impact since she's trained for it, whereas a witch or another magical girl will be at more of a disadvantage. Moving at these speeds also allows her to simply avoid fighting if she wishes, and I think we can all agree that the best way to avoid taking damage is to no longer be in a fight where you might take damage.
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TL;DR: Maths, therefore Sayaka should stab things at about Mach 0.3 to Mach 0.6, assuming standard sea level conditions.
