Volatility is the main concern [1]. Making trading decisions based on microseconds is speculation. How can anyone or anything reach an understanding of something as complex as markets in this short time?

Other concerns include unnecessary waste and unfairness [2]. The point is that this is a zero-sum game that makes us invest in infrastructure that will soon get obsolete once someone else builds a faster infrastructure.

Fast trade isn't "efficient". Long-term stability is.

[1] https://en.wikipedia.org/wiki/2010_flash_crash [2] https://en.wikipedia.org/wiki/Flash_Boys

It is a speculation, yes. That does not mean it is not useful, or is harmful to the society.

How is it useful?

People that need to sell something will find a buyer faster.

> How can anyone or anything reach an understanding of something as complex as markets in this short time?

Computers mostly.