Real investment decisions - as in, decisions of which factories to build, which products to develop, who to hire, and so on - are made on a time-scale of weeks or months. Having the stock market react on a sub-second time scale certainly has no effect at all on those decisions.
To gain some perspective, consider if trading operated on an hourly-auction type system. The stock price would react slower and in a different manner that it does today, and that could change the income distribution among traders. But honestly, why should anyone care?
When it comes down to it, there is no upside to trades going that fast to anyone outside of the myopic circle of professional traders. Even those professional traders only have an upside as long as they participate in a vicious arms race. However, there are downsides.
One downside that many people argue is that the resulting distribution of income is not just, because HST firms suck a revenue stream out of the economy without building anything real.
The more important downside (in my opinion) is that HST sucks intelligent people away from sectors of the economy where they would positively contribute by developing and improving directly useful technology.