I don't want to get caught up in arguing what is noise in the mathematical sense. I think the parent is refering to the fact that the arbitrage traders are not adding any real value by making these trades this fast. The market would adjust in miliseconds. Who really wants to let some guy who built a 825 mile cable be a rent seeker on every trade for all eternity? https://www.businessinsider.com/chicago-stock-exchange-respo...

He's not a rent seeker. Absent his existence, the people on both sides of the trade would be harmed by crossing a bigger spread.

Noise compared to the signal actually needed for a market to function. Markets functioned nicely on much, much longer timeframes in the past.

They also had much higher transaction and information costs.