The difference in intelligence between an egg and a chicken aren't just reaction times. Basic cellular life might respond to stimuli in a way we can call "pain", but they'll never understand it the same way.

The difference is like us touching a hot stove and pulling away as an automatic response, versus the various forms of torture we've invented.

That doesn't mean it's right, just that there is a difference and we'll all draw a line somewhere. A bit like how people see eating cats and dogs as somehow different from chickens and cows.