"Extremely fast" doesn't have any absolute definition. We were talking about human visual perception, so "fast" in this context could reasonably mean "fast enou

"Extremely fast" doesn't have any absolute definition. We were talking about human visual perception, so "fast" in this context could reasonably mean "fast enough to have no visible smoothing". A typical LED is much faster than that, so in the context of fake candles I think "extremely fast" is fair. In the context of data transmission it would mean something else.