Idealogy makes less difference to economic performance than politicians like to think. Worse, new policies -- even the best of policies -- sometimes take effect only slowly. Take the interesting case of Jamaica, whose voters turned out a bankrupt socialist government under Michael Manley four years ago. His successor, Edward Seaga, is a conservative determined to return the country to the rules of the open market.

Are things better? Not much, not yet. Events in Jamaica can tell you a lot about the trials of a small country that now stands at just about midpoint in the range from the world's poorest to the richest.

The voters were right to reject Mr. Manley, not because he was a socialist but because he was a persistently unrealistic one. He incited a capital flight and couldn't end it. He borrowed desperately to maintain a subsidized standard of living for the country.

The remedies were pretty obvious, but they haven't made Mr. Seaga popular. Earlier this month, the latest round of de-subsidizing resulted in sharp increases in the price of oil and, in response, riots in which seven people died. As Blaine Harden reported in this newspaper, the unpopularity of Mr. Seaga's austerity is strengthening Mr. Manley again.

Jamaica's most urgent economic need is investment capital. President Reagan has tried to encourage American companies to invest, but it goes slowly. Reversing a capital flight is a difficult feat, and it's going to take time. Meanwhile, the country staggers along under the debts of the 1970s, and the bauxite business isn't improving. A better bet for Jamaica is agriculture, where there are now promising indications of growth.

But there's more to it than economics. Jamaica is a small English-speaking country not far off the coast of North America, and there is hardly a Jamaican family that does not have relatives in the United States or Canada. Jamaicans are extremely well informed on the style of life in the rich countries, including the availability of consumer goods and government benefits. If people in Jamaica work as hard as their cousins in Washington, why shouldn't they live as well? The answer is that they are living in a society whose infrastructure -- from the roads to the phone service to the school system -- is not as strong. They can be strengthened, but only gradually.

Jamaica's greatest resources are not the bauxite mines but levels of education and public health that are among the highest in the Caribbean. The country is equipped to achieve rising prosperity over time -- if Jamaicans have the endurance to resist frustration and impatience. That's the choice around which Jamaican politics is now revolving.