Sakura got no sudden boost in speed because she hit Kaguya. That's completely forgetting the fundamentals
Byakugan
The primary ability of the Byakugan gives the user a near 360º field of vision, except for one blind spot at the back of the neck above the first thoracic vertebra
You must be registered for see images
Sakura was left around this position during Kakashi's offensive strategy
It's for that reason Kaguya failed to notice Sakura in time(you can notice the exclammation mark):
You must be registered for see links
Not to mention that Kaguya's only threat are Naruto and Sasuke because they can seal her, so someone like Sakura is someone she wouldn't pay attention of.
Limits in reflexes
A reflex is an involuntary and nearly instantaneous movement in response to a stimulus(a detectable change in the internal or external environment).
There is a limit in every one's reflexive capabilities.
Example: - Even for the most reflexive human being, it's impossible to completely dodge 6 tennis ball coming from different directions splits of seconds after the other even after awareness of all of them
Kaguya was coming out of a 5 counter-strategy battle before getting hit by Sakura
1- Opens the portal to try an escape
2- In impossibility to enter the portal, she changes tactics and attacks Naruto's clone(not knowing it's one)
3- Noticing this, she again changes tactics by attacking the one on her right through the portal, but Kamui close the portal
4- Noticing this, she now decides to switch to ice dimension, but Sasuke was preparing Amaterasu for that
5- Noticing this, she now decides to jump away from the duo
And it's after coming out of all this that a Sakura coming from her blind spot out of nowhere actually managed to land the hit. Notice the position of Naruto/his clone/Sasuke only slowly changed compared to the initial position when the strategy started(after Kakashi destablized her balance), so everything was happening in splits of seconds, yet Kaguya mindly and bodily reaction to 5 actions happening in this split of seconds in a row, only failing on the 6th from an action she almost failed to notice
So no, Sakura didn't get any boost in speed and her hiting Kaguya means close to nothing. Put her in 1on1 and Sakura would have gotten stomped U_U