Weird phrase 
In the introductory passage there is a sentence One of the main issues regarding high frequency trading is the difficulty in determining just how profitable it is. What does it mean? Well, I can understand that it may be difficult to determine the profitability of high frequency trading, by why is it a main issue? Note also that this is an article on algorithmic trading generally, not specifically high frequency trading. Furthermore, this confusing sentence is followed by another one about the income generated by some 300 hedge funds and other participants. I have no idea what to say about this amount - is it a lot of money or not. Also anyway, if we have this number available, then why is it a difficulty to determine the profitability of HFT?