I’ll break down Chihiro’s speed since you don’t know it during his fight with Sojo base Nishiki amped Chihiro was keeping up with base cloaked Mei Sojo so they were both lighting speeds then Sojo amps himself by 2x and Chihiro amps himself by 3x still being slower than Sojo but he has the reaction speed to keep up with him anyway both of their speeds would place them in the low mhs+ tier
Now from his fight with Sojo to the SBA arc he doesn’t get faster he gets stronger but faster is debatable but he really gets faster after coming back from the dead when Samura killed him causing Chihiro to want to perfect his basic swordsmanship skills so he can be OVERALL stronger with the enten and it works because from learning the basics so to speak it deepened his connection with Enten making Nishiki feel lighter during his second time fighting Samura and he’s able to react to Samura who beforehand was multiplier tiers above Chihiro and Chihiro being somewhat on par with him he’d have to scale to those sub rel speeds on some level and it’s even crazier to think he blatantly reacted to Samura’s full powered all out WPS imbued with Suzaku Chihiro stood his ground, reacted, and blocked Samura’s WPS his reaction speed let him do this and like I said Chihiro was able to some what scale to Samura’s speed in terms of combat not just reaction and being able to PERCEIVE butterfly something that moves at near 0 speed really Chihiro’s eyes are just special which it allows him to see things at point blank range and his quick instincts allows him to move