The boundary is "arbitrary", but clearly there has to be one, otherwise we're all nodding in agreement as trading speed heads inexorably towards Plank time units.

We have some convenient "lines in the sand" that we can use as a guide:

- To make trading fair globally, the round-trip time for light around the planet could be multiplied a couple of times. That's about a second.

- The fastest possible time a human can parse the meaning of a long headline (not the full news article!) is... about a second.

- Nobody in their right mind should be buying any significant volume of shares without double-checking their order. There's no way to do this even vaguely carefully in under... a second.

Etc...

Bots trading faster than a second are trading with each other, and the only signals they have are each other.

Humans are what markets are for, not bots.

This reminds me of a story from WWII where a bunch of generals took a holiday at the same time, leaving a junior general in charge. He was in a bit of a panic because he was expecting to be overloaded with work... but found it easy. The generals were making work for each other by requesting reports, organising meetings with each other, etc...

Bots make work for bots, they generate signals for bots ever faster, to be processed by faster bots still, etc...

It's just... nonsense. Zero real information is being generated, they're just "riffing" off of the much less frequent human-initiated trades, all of which take minutes to organise and execute with due diligence.

It's like being asked to write a 10-page essay on a three-line poem.