Quote from Cutten:
For a perfectly efficient market, it does require perfect competition. The efficiency is relative to the level of competition in the market in question.
E.g. ""An 'efficient' market is defined as a market where there are large numbers of rational, profit-maximizers actively competing, with each trying to predict future market values of individual securities, and where important current information is almost freely available to all participants. In an efficient market, competition among the many intelligent participants leads to a situation where, at any point in time, actual prices of individual securities already reflect the effects of information based both on events that have already occurred and on events which, as of now, the market expects to take place in the future." - Eugene Fama
It is competition from rational profit-maximisers with cheap access to market sensitive information that creates efficiency. Therefore any barriers to that process will degrade efficiency. For example - irrational or non-profit-maximising behaviour, expensive or hard to find information, limitations (e.g. low capital) on the ability of profit-maximisers to trade on superior information, small numbers of competitors, and so on.