That's the wrong question. The fact that the markets make no distinction between speculation and investment is part of the problem.

And yes, buying and holding would slow down the market. A two to four week period would be about right, as a random guess, to eliminate predatory shorting, exploratory non-trades and pumping and dumping.

If traders don't care enough about the companies whose shares they trade to keep them for that long, they can always go find something useful to do.

As for computers and speedy trading - just because something can be done, doesn't mean it should be done. No one has a problem with the illegality of using a botnet to trawl for credit card numbers. And yet for some reason anything that happens on a stock market is supposed to be immune from legal oversight - why, exactly?

At the very least HSTs could be taxed. The fact that they're not, and the fact that they've been allowed to happen at all, is a political problem and not a technological one.