We have the quote about Candice Cattnip's “Electrocution”, which talks about the technique as:
Although her might has significantly weakened now that she has lost the power of her Vollständig, the blow from her “Electrocution” still far surpasses any lightning in the realm of nature.
This seems to be referring to power, but we have another quote:
The thunderstorm which even surpasses nature’s lightning, was mostly absorbed into the ground through the large tree, meanwhile the flank attack generated by the thunderstorm was repelled in the same fashion as before by the big sword in the possession of the man with the black jacket.
It does not seem to be about power, but only in general. Thunder clouds and sounds are referenced several times and we still have a Giriko speech that supports this:
Hmm, although it is often misinterpreted, the speed of lightning is far less than the speed of light. However, since it is influenced by factors such as air humidity and atmospheric pressure, those numerical values have a tendency to change. Good heavens, speed is truly an ambiguous thing compared to the flow of time isn’t it?
So I think Candice's Electrocution should match the lightning's return stroke, which is about 100,000 km / s (Mach 291545). So we have Relativistic speed for Candice's attack speed.