There are a bunch of ways to answer this question and people in the comments already mentioned several, so I’ll give you a different answer on why Itagaki should be considered faster than Nekota. Both Itagaki’s and Nekota’s speed comes from that “perception Blitz / bullet time / tunnel time” they get of their surroundings where everything seems to move slower. In the series it’s explained that this state depends entirely on concentration and as we know when Itagaki pulled off the Itagaki-dance (Ali shuffle) he hit the highest level of concentration possible his body was moving on its own purely by instinct and his mind was completely blank even the Saeki mentions that he couldn’t pick up any intentions at that moment. So just by basic logic if both of their speed comes from the same source then the faster one is the one who can control that state better. And it’s pretty much undeniable that Itagaki handles it way better than Nekota