Input and output pipes have a much bigger fluid capacity than regular vanilla pipes, they can hold 250 units of fluid instead of just 10.
Also they have special properties, using a feature of vanilla fluid mechanics: If a factory input pipe is placed into a fluid network, it will very quickly suck up all the fluid it can get and fill up, even if the fluid level in the rest of the network is very low. Similarly, a factory output pipe will eject all the fluid it has into the connected fluid network.
These properties mean that every time the connection updates, it transfers much more fluid than it would with normal pipes, and that in turn means that you can use a much lower update rate than you'd need otherwise.