That quote is about her Heilig Pfeil, the Reishi arrows, not her lightning strikes. You're being misleading by leaving out context:
Although it is far slower than an actual lightning strike, this arrowhead of Reishi still came flying at a rate worthy enough to be called 'lightning speed'.
They dodged it just in the nick of time, but if they had been caught up in the electrical discharge, there's no way they would have gotten away without consequences ― before he knew it, Ginjō was brandishing a large sword in his hands, sweeping it sideways to disperse the lightning strike together with the Reishi that filled the air.
Whilst readying his large sword, pointing it in the direction in which the arrow that mingled with lightning flew in from, a daring smile played on Ginjō's lips.
"Ha… awfully slow for lightning wasn't it? It seems impossible but, that couldn't possibly have been a lethal assassination technique just now could it?"
You're also being misleading for leaving out the context where Candice address that her Heilig Pfeil are slow and shows how her actual lightning is fast:
"Who are you calling… slow?"
It was the voice of a young woman.
Meanwhile, a Reiatsu different to that of Shinigami or Hollow, swelled up at the center of the group.
Was it that she had cast off some sort of special coat that implements active camouflage, or perhaps she was using a different method altogether? The woman who materialised so abruptly as if she had seeped out from the shadows, was completely wrapped in white plasma.
Her white western style garments which were mostly open at the chest, was designed with her unique modifications applied in which a large portion of her abdomen, including her navel, was exposed.
Scars which appeared to be the result of some experimental procedure could be seen all over her body including parts of her exposed skin, however Ginjō had no time to pay attention to small details.
Then, the woman who had emerged from the shadows shouted at the top of her lungs in a voice mixed with mockery and irritation.
"If that's the case…… I'll let you assholes die quickly!"
With those brutal words, small bolts of lightning extended into the sky from her raised hand —- a genuine lightning strike not entangled in an arrow came rushing down towards Ginjō and the others with a thunderous rumble.
The bold speaks for itself and answers how the lightning is different than the Heilig Pfeil.
