Actually the commissions are the key factor, if you want to succeed in HF trading. Let's say, you would do 500 r/t per day and you would pay 1 cent spread for the 10000 shares, this would make 100$ in fees per r/t. Of course, it would be better to have to pay a flat fee around 6$, per side. But even with a flat fee, at the end of the day and after 500r/t, the broker would have won 6000$ in fees and your p/l could be at zero. You would do this HF trading for a day, a week and after a while be miffed when you saw how much good you did for your broker (but not necessarily for you).
First of all, you need an edge. Going long just because a stock just went up is not an edge. You may think you can take a penny or two but it's not that easy. If it was, we'd all be doing it and making an absolute killing without any exposure 10,000 times a day.