Reflexes and Shuushin get confused all the time.
The sharingan can predict movements, while itachi's eyes in particular can read souls and give you fluid movements (straight commas).
Itachi's speed is nothing more than above Hebi Sasuke.
His reflexes and body movement goes in hand with Kcm naruto.
Only Madara and Sasuke were said to have straight commas if I remember correctly, most likely is the EMS since both had it and that's what Sasuke was using when Madara said it. Hebi Sasuke was only slightly slower than Itachi but was still keeping up to Itachi speed and react to his attacks while Sasuke couldn't even react to A's speed even though his MS could track A's movements. Orochimaru said he sensed power in Sasuke's eyes that surpass Itachi's in part 1 so Sasuke's MS would be stronger than Itachi's.
KM Naruto gets his speed from Kurama chakra yet Naruto only had 1/13th the power since he made 12 clones before meeting up with Itachi and Nagato so Naruto was keeping up with Itachi's speed when he was weaker than his full power. Plus Naruto was in intense training where he was injured and took no breaks during that training then entered the war. KM Naruto full speed is compared to Minato's FTG as you only see a flash when he goes full speed.
Also A more than likely faced Itachi and Kisame as they knew what the MS was and compared Sasuke's Amaterasu to Itachi's. This is how Shi knows lightning armor level 1 can't be read with the 3 tomoes and lvl 2 counters MS.
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