What happens in a half-second of high frequency trading activity? The answer is a lot more than you'd guess or notice.
This video illustrates an actual half-second of trading in in Johnson & Johnson stock, slowed down so it takes five minutes to watch. Don't worry, you don't have to watch more than a few (slowed down) seconds to get the point. The size and scope of what is happening changes the game and the playing field.
The question isn't whether high frequency trading is about efficient allocation of capital or attempts at short-term market manipulation ... it is what you decide to do after you know it is happening and becoming the norm (rather than simply an understood but infrequent risk).