There is (perhaps) no way a human can compete in the stock market any more. This is the age of AI, where computers are pushing around billions of dollars in an automated fashion. Taking every arbitrage advantage in milliseconds, and are even able to find under- and over-valued stocks in the blink of an eye.
It's possible to invest in the stock market with very little trading. The GP's comment spells out the best strategy--use index funds to spread risk and reduce expense, and invest for long time horizons. This is still a great strategy.
It's also possible to try to turn a short-term profit by trading individual stocks--often called day-trading because you clear your positions within the time period of days. High-frequency trading has greatly reduced price spreads, which makes it harder for the average guy in his underwear to compete. This is not investing though--more like gambling.
For more on how high-frequency trading really works, I recommend:
These high speed investors aren't drastically modifying the value of the market. If they were they would be creating arbitrage situations that would be closed right away.
Whether or not they're drastically modifying the value of the market is really hard to say, I don't think we know that fully. Honestly, I don't think we understand it that well yet. You can make some fairly reasonable assumptions that it doesn't cost the typical investor that much over his investing life though. I think part of those assumptions are that the high speed traders are simply trying to move faster when they acquire knowledge though, to me, that seems kind of elementary for the guys that invented all these exotic synthetic derivatives and ways to mask risk and shift it around... You couldn't tell me that they aren't trying to think up other uses for those technologies if they exist; like maybe you can measure what counter parties are doing with high performance timing and get some insight.
Fundamentally, are they leaving that few dollars on the table or are they picking them up? It may only be a few dollars but it's a big difference between paying it and taking it.