I suppose in this case, in a world of Swords and Sorcery, super human speed is kind of unfitting.
But I suppose when it comes to roleplaying Mizuki's speed it could be fixed with some finer execution and clarity. Not totally getting rid of her aspect.
Because while reading those posts I would never had known she was going at the speed at light, but with better execution this whole mis-communication could of been avoided.
Maybe she could find a compromise? Maybe wearing some kind of magic amulet allowing her to vibrate extremely fast, granting her speed.
It could even be interesting RP during battle, vibrating in place to dodge through an arrow. I dunno, just think that we shouldn't be quick to disregard an interesting RP aspect that could be cleaned up to work within this universe. Not just totally ignoring it.