When President Reagan signed a $1.2 billion bill last August to pay $20,000 to each Japanese American forcibly interned in U.S. camps during World War II, the measure clearly was intended to pay the money before all the surviving claimants died.

Yet the payment process is so slow getting started that time and age may eliminate many eligible recipients before they ever see the money.The internment camps were set up in early 1942 after the Japanese attack on Pearl Harbor and some 120,000 persons were detained there - in violation of constitutional rights - until World War II ended in 1945.

In the 43 years since then, half of these internees have died; only 60,000 remain. Most of the remaining survivors can't be expected to last too many more years.

Yet the schedule set up by the administration only calls for $20 million to be disbursed in 1990, the first payment year. That would compensate only 1,000 of the 60,000 Japanese Americans. At that rate, all would be dead before very much of the reparations money was ever allocated. The legislation signed by the president allows up to $500 million a year to be appropriated.

The administration isn't deliberately delaying payment as a way to save money. Officials say the law requires that the oldest survivors be paid first and it takes time to verify all these people and sort them out by age. Once that is done, payments are scheduled to rise to $170 million a year, about 8,500 persons a year, or enough to complete the reparations job in seven years.

Certainly, there is some justifiable start-up delay, but once that is out of the way, the payment schedule ought to move as quickly as possible, perhaps finishing the task in three or four years.

A great many persuasive arguments were made against paying reparations at all, since they amounted to an across-the-board token at best and the U.S. government had officially apologized many times before. But once the issue was decided, it should be dealt with quickly.

In this case, because of the age of most people involved, justice delayed would be justice denied - permanently.