IT'S "TIME TO MOVE'' to a United Nations arms embargo on Iran if that nation keeps rejecting a cease-fire with Iraq, said Secretary of State George Shultz, heading off to Moscow. The going was apparently slow in his talks on this issue with the Soviet foreign minister. But he's right. Soviet readiness to address the embargo anticipated in the United Nations Security Council's cease-fire appeal of last July is a fair test of the Kremlin's vaunted ''new thinking.'' The United States is pushing hard for an early council vote by the end of the month.

For years the Soviet Union, attentive to the sensibilities of an important country on its border and perhaps seeing an opportunity to pick up a major loose piece on the geopolitical chessboard, held off from putting pressure on Iran, which had thrown back the original Iraqi invasion and itself invaded Iraq. But gradually Moscow has come to appreciate the disruptive effects of Iran's Islamic revolution on the whole Gulf region and the potential impact on the Soviet Union's own large Moslem population. In the last year, it has flirted with the idea of cooperating with Washington to try to halt the war, but it has resisted moving to the key step of voting sanctions against Tehran.

Why might it stop playing for time now? The United States makes a compelling case that Iran has been abusing international patience and refusing to negotiate in good faith. The U.N. resolution calls not only for a cease-fire and withdrawal to the border (of obvious interest to Iraq) but also for a body to determine responsibility for starting the war (a point of pride and a possible face-saver for Iran). Then, too, Iran may be tiring. Draft dodging apparently is common in Iran, and the government has been unable to collect the large number of young recruits required for its long-trumpeted offensive to capture Basra. In what appears to be an emerging permanent stalemate, the Soviet Union may find Tehran readier to heed pleas for a fair settlement, especially if there is a cost to turning a deaf ear.

Granted, West and East have had plenty of occasions to learn not to count on Ayatollah Khomeini to act by traditional political logic. Regardless of his response, however, the Soviet Union has something to gain from joining a broader diplomatic effort, and something to lose by not joining it.