They also mentionned his actual speed.

I guess they could always tell us he's only having a nice walk at 200kph, for stamina purposes too, but he has great short distance/reflexes speed.
Regarding reflexes, part of it is the instinct where the line of fire will be. Which gives a lot more time to do something. Then add some nen into the mix and you get the idea.

Regarding Killua's bullet, you have to remember these are air propelled. So they are a lot slower. Also they are fired at a greater distance, which makes all of the line of fire detectiong easier for a nen user like him.

There's still a lot of bullshit, but at least you can think of those ideas to keep faith in that show