As the Ottawa "summit" fades into history, the reviews are as interesting in their way as the performance -- or the performers' own review in their final communique.

Some critics succumbed more than other to what must have been the biggest brainwash ever organized by an American delegation to these affairs. But the consensus seems to be that the presidents and prime ministers got along well: that despite their doubts and deep differences the atmosphere was cordial, the exchanges decorous, that for President Reagan it was a boffo show of long overdue "American leadership," that out of it the seven leaders emerged with a sharper sense of "common purpose."

But a sense of common purpose in general is no substitute for the capacity to act in concert on particulars, and there was little of that capacity in evidence at Ottawa.

As for "leadership," the test is always whether -- and how fast -- anybody is following. In this respect, Henry Brandon, of the London Sunday Times, as well seasoned veteran of these spectacles, had it just about right:

"If the Ottawa summit achieved anything beyond a better personal understanding among the leaders of each other's viewpoints, it was that it induced the individual ships of the alliance convoy to steer in a more orderly manner, though a distinctly different speeds. But beyond that, basic differences generated by different national interests, stubbornly persisted."

The convoy metaphor could hardly be more apt. For the first law with convoys is not that they steer "in an orderly manner" but that their members do not said at "distinctly different speeds." Safety lies in sailing at the speed of slower ships so that laggards are not picked off.

That is the problem that Ottawa did not -- and could not -- solve. And nothing illustrates it better than the relative speed displayed by the United States and West Germany.

Reagan's forced-draft rearmament would spend $1.5 trillion for defense in five years. But his economic policy, with its high interest rates, is making life miserable, financially and politically, for the West Germans and other allies. Reagan promised that his economic experiment will work so well that interest rates will just naturally start falling later this year. But Germany's Chancellor Helmut Schmidt was not impressed.

More important, Schmidt arrived at Ottawa prepared not be impressed. Before departing Germany, he had told an American visitor exactly what his post-Ottawa strategy would be. He would hear Reagan out. Then with this conclusive evidence of the president's intransigence as his best argument, he would return to Germany and cut the budget as the only way to solve economic problems.

And he would cut across the board -- defense spending as well as social welfare spending -- because nothing else would be politically acceptable.

That's what he's done. That is to say, when American "leadership" is demanding that the allied convoy speed up its efforts to redress the military balance with the Soviet Union and the Warsaw Pact, the West Germans, whose position as pacesetter is critical, are slowing down and falling further behind.

The West Germans are not alone. Other Europeans (including other NATO members like Belgium and the Netherlands who were not at Ottawa) are increasingly vulnerable to being "picked off" by their own public opinion -- by rising neutralist sentiment, including movements for unilateral disarmament.

Over time, this could raise some interesting questions for American public opinion as well. The full effect of the new Reagan defense program on U.S. spending has not even begun to be felt. But it includes an extraordinarily costly effort, for example, to create a rapid deployment force and an American military presence to protect European and Japanese as well as American interests in the Persian Gulf.

Pleading the geographical constraints of NATO among other things, the Europeans are willing not only to deploy forces to the region but also to take up the slack by increasing their NATO contributions. The Japanese have their own internal political reasons for hanging back.

Not just the unsoundness but the inequity of all this a bound to sink in over time. The same may be said for the question of European willingness to accept the theater nuclear weapons that the Reagan administration has been trying to force upon them. Ditto for the European insistence on promoting East West trade, while the Reagan strategy call for turning it on and off as an instrument for influencing Soviet policy.

In time, some shrewd observers of the American political scene expect that the wisdom of keeping more than 250,000 of America's best troops on the European front may be called into question. "Leadership," in short, can all too easily turn into isolation, or even isolationism, when nobody's following.

Ottawa, for all the importance vested in it by the presence of seven top leaders and 1,500 news people, created no new spirit to deal with this disturbing outlook. To the extent that it generated serious contemplation of it, Ottawa may have made things better. But to the extent that it contributed to an illusion that the outlook is somehow not disturbing, it may have made things worse.