The securities markets are open to all -- including "investors" (i.e., those who buy with a "long-term" horizon, whatever that is these days) and speculators. The speculators engage in what is called arbitrage -- finding discrepancies in the market and taking advantage of those discrepancies to make a profit.
It can be argued that the speculators serve a useful purpose by keeping the markets "efficient" -- i.e., making sure that prices don't get too out-of-whack.
To a large extent, this problem was caused in the late 90's when the SEC mandated that stock prices be quoted in pennies rather than the quaint eighths, sixteenths, etc. (of a dollar). This led to traders being able to game the system with minimal risk by "stepping in front" of published (and presumably real) quotes for a dollar (i.e., a penny times 100 shares, which is a "round-lot" order).
The idea of decimalization was that it would benefit the small investor, but in fact most small investors (i.e., you and me) invest through mutual funds or something similar, typically managed by an "institutional investor" (e.g., Fidelity). The narrowing of the bid-ask spread made it much more difficult for institutional investors to find large blocks of liquidity without "market impact" (what happens when other traders find out if there's a big buyer or seller in the market).
There are markets are designed for institutional investors and that don't exhibit this behavior -- one is a company called Liquidnet, which only allows institutional investors and specifically excludes the "sell-side" traders because of the problems mentioned above.
There is a lot more to it, and yes, the markets continue to get faster, with "co-location" being an important factor now (because the speed of light is "too slow", many traders locate their computers in the same data center as the exchanges to eliminate transmission delays). Some traders boast of microsecond and even nanosecond latencies, but for the most part they are cooking those numbers to make them look better than they really are.