https://www.sec.gov/comments/s7-02-10/s70210-122.pdf
Choice quotes:
As the number of trading venues increases, discrepancies in prices across those venues will naturally result. The price discrepancies across multiple markets create an opportunity for nimble traders to make a small arbitrage profit by scouring the markets for these discrepancies and eliminating them. As the number of trading venues expands, the number of such arbitrage opportunities increases. So, it is not surprising that we have seen a tremendous increase in trading volume over the past decade, and that the activity is increasingly dominated by "high frequency traders." While Vanguard does not engage in this type of trading, we recognize that such trading has a positive impact on the markets at large, including longer term investors. Such arbitrage trading enables investors to get a fair price across market centers. Vanguard believes that the market structure changes facilitated by the Commission's various regulatory initiatives and the "knitting" together of the marketplace by "high frequency trading," have led to a significant decline in transaction costs for long-term investors over the past ten years through increased liquidity and tighter bid-ask spreads.
and
Various groups have attempted to quantify the reduction in transaction costs over the last ten to fifteen years. The Commission will continue to receive this data throughout the comment period. While the data universally demonstrate a significant reduction in transaction costs over the last ten to fifteen years, the precise percentages vary (estimates have ranged from a reduction of 35% to more than 60%). Vanguard estimates are in this range, and we conservatively estimate that transaction costs have declined 50 bps, or 100 bps round trip. This reduction in transaction costs provides a substantial benefit to investors in the form of higher net returns.