Exactly. Someone could pull a gun on me as I'm walking down the street. Visually I could take in him doing that as it happens, but that doesn't mean I could actually react fast enough to do something about it. In her case, even if you take her off guard by pulling a gun on her, near instantaneous reactions would allow her to still be able to react, even if her brain processing the image wouldn't be that much greater than mine.

For characters reacting super duper fast like in DBZ where they hit each other in flashes, I assume some sort of time dilation is going on where everything they are doing appears to be normal speed to them but not to every else.