A market is efficient if profit opportunities are eliminated almost instantaneously.
It works like the queues at the pay points in a busy grocery store. The waiting time for each queue will be more or less the same. If one queue is much shorter than the others, people will quickly fill up the queue.
This applies to a large extend to the Stock market and even the Forex market. There are thousands of people on the lookout for a good deal. When a good deal comes along, they jump at the opportunity. Greater demand push up the price until it is no longer a bargain.
Karl E. Case, Ray C.Fair, Principles of economics. Seventh Edition, Pearson Prentice Hall, 2004, Chapter 1