Why is a democracy considered the most inefficient form of government?

One problem with democracy is that it's hard to plan in a long term (longer than 4-5 years usually), as usually people get dissatisfied or bored with the current party and tend to vote for another one, thus it is typical of democracy that the parties rotate in the position of power. In this situation any changes introduced now could be reversed by the other party in 4-5 years, which makes any planning hard.