You don't have to be so much faster than your opponent to do this. You can just be good at dodging.
Too bad,
it's accepted! Do a separate CRT to fodderize both Sans and Chara if you wish so, since what you're talking about is not the direction Reaper meant to go with.
Especially with Sans's knowledge on game mechanics, a predictable attack would be even easier.
Game mechanics are not canon, at least on this wiki.
As for Sans not predicting you would attack him after falling asleep, he mocks you for that split second for "really thinking" they could hit him.
And? I don't see how it supports your point at all. You do
NOT "predict" an attack while asleep, and "didja really think" does not imply that he predicted anything.
And again, make your own CRT on that to remove Sans' reactions rating and Instinctive Action.
Narratively Chara is just possessing Frisk's body. So yeah, they used their attack speed to tag Sans. Nothing suggests they amplified them or anything.
Before Chara's intervention: 24 attempts, 0 hits.
After Chara's intervention: 1 attempt, 1 hit.
Hmmm, yeah, nothing special!
Chara breaks the rule by attacking twice, so yes he's off-guarded.
It's not canonical rule of the game, unless you do (yet again!) a separate CRT that enforces "turns" system as canon and thus Omega Flowey, Chara, Sans, and maybe a couple of others get Law Manipulation. Without that, it's not canon thus dismissed.
In terms of Instinctive Action, even Sans' justification for it says right after "only getting hit when Frisk broke the game's combat system"
Outdated and should be changed. The real reason is here:
with
higher reactions (
Capable of consistently dodging Frisk to such a degree that they could never land a hit on Sans, even if they reset and knew exactly what he would do and where he would be, and even if he was asleep. Was only hit after
Chara interfered)
which goes into that same notion that Chara didn't blitz Sans, just caught him off guard by going against the game's rules.
Turns are not canon, make your own CRT, etc.
I don't think it's hard to imagine that Sans after getting in the habit of dodging Frisk multiple times, that when Chara steps in breaking the rules by striking him two consecutive times when Sans thought it was impossible, rather than Chara amplifying Frisk to be able to blitz foes.
Turns are not canon, make your own CRT, etc.