Theory is the faster the market is, less the room for arbitrage, and better for the day trader. HFT has certainly narrowed the spread. Big problem currently is

Theory is the faster the market is, less the room for arbitrage, and better for the day trader. HFT has certainly narrowed the spread. Big problem currently is moving large block (institutional) trades without affecting the market. Thats why a good chunk has moved to dark pools. (I work for a dark pool ATS).