For several years, the Wall Street wizards who built a faster, more fragmented stock market justified their creation by pointing to the benefits it yielded for investors in the form of lower trading costs.

But as the speed and complexity of the markets have continued to change at a rapid pace — with trade times now measured in millionths of a second — a growing number of studies and market participants suggest that those benefits to investors have stalled or even started to reverse.

Research from the broker Abel/Noser indicates that the total cost for an investor to get into and out of a single share of stock fell by more than half between 2000 and 2010, to 3.5 cents. Since then, though, the cost has leveled off and then ticked up in the most recent quarter to 3.8 cents, confirming a trend that has also been visible in recent data from Credit Suisse Trading Strategy and from Celent, a consulting firm specializing in financial markets.

The advantages of the nation’s increasingly high-speed stock market are under the microscope after a number of recent trading malfunctions underscored the risks and instability that have come with the rapid changes. This month, one of Wall Street’s most important trading firms, Knight Capital, lost $440 million in 45 minutes after installing faulty software designed to keep up with an evolving market.