Tsunade is not only taijutsu. She is also intelligent, highly observant and analytical, allowing her to see through most deceptions as well as sometimes making better strategies than the advisers. Take example Pein's arc and now, Madara fight.
Jiraiya commented that no one could stand equal to her in terms of combat and medical prowess.
Many say that she is slow, but once again, as I said her reaction speed is even on tier with Naruto and A (2 of the fastest characters in Narutoverse)
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If she can keep it on tier with those two, what makes you think she can't tank Itachi's speed on hand-seals?
im not saying shes not, even though im not quite sure what youre referring to in the madara fight, shes just brawling and healing her team mates, which is expected from her skill sets, but itachi has demonstrated superior analytic and intelligence feats. i doubt you can provide evidence to the contrary, even with jiraiya's comment, which im going to have to go back and look at the context off, its clear shes not unparalleled in battle, you would have to have fanboyish tendencies to think otherwise. medical wise its known shes the best, no one disputes that fact, but shes demonstrated no particular skill with genjutsu that would lead me to believe she could combat itachi's main skillset
and observing a fight from a removed 3rd person stand point and being able to follow the movements is not the same as being on the same tier with them in speed or reaction time, that said itachi has hand seals so fast the sharingan has failed to pick them up on several occasions, there's no way tsunade has the physical capacity to out pace itachi's handseals with a reaction or react fast enough to out pace the sharingan movement projection, dont forget ororchimaru was the prodigy of the sannin 3 and itachi fodderized him, not saying a<b<c logic applies because it doesnt, but you cannot ignore how efficiently he can utilize his techniques