London opera fans last month heard some of history's fastest and strangest performances. It all began when the musicians in the orchestras at Covent Garden and the English National Opera announced they were only going to play three hours a night.

Now, if you take a fast look at some of your favorite operas, you will find that many - "Traviata," "Rigoletto," "Boheme," "Tosca," "Trovatore" and a good many more - can be played on your phonograph in two hours or less. But that's without time out for intermissions, changes of scenery and costumes and a chance for the singers to rest between strenuous scenes.

In those London opera house, whole acts were cut, intermissions were shortened or just plain skipped and there was some very fast and fancy improvising of scenery when there was not time enough to shift big sets. and costume changes? Grace Bumbry, the American mezzo who was singing the Princess Eboli in Verdi's "Don Carlo," said, "I was so busy changing costumes between Acts Two and Three and I had no time to take curtain calls." When an opera house tells its stars they cannot take those ego-boosting trips in front of the curtain, that house is headed for serious trouble.

If you listen to Metropolitan Opera broadcast, you know how these popular two-hour operas get stretched out to three, even four hours. It's those intermissions. Eliminate some or all of them and you save an hour's running time. That's one thing they did in London for three weeks while the negotiations with the musicians went on.

The intermissions were not, however, the only things that got meddled with. Imagine a conductor, determined to break the track record in "Boheme," reminding the tenor that poor Mimi's hand is terribly cold and suggesting that they ought to move "Che gelida manina" along at something like a presto, so the hand does not freeze completely before they finish.

And why shouldn't the beleaguered house offer a prize to the conductor who rips through "Aida" in the shortest time? Cuts would of course be permitted. It would be just like the days when Alfredo Salmaggi put on operas at the Watergate, just below the Lincoln Memorial. The rule was that if half the opera was finished before it started to rain, no refunds were necessary. Well, I want to tell you that you never heard a Triumph Scene in "Aida" such as the one performed the night that era's Gordon Barnes predicted rain at "about 10 p.m."

About 10 minutes before 10, the lightning was flashing and the distant thunder was not so distant. The conductor on the orchestra barge was driving those singing Egyptians and Ethiopians along at a speed that would have won them the Preakness. He went so fast that the trumpets couldn't handle Verdi's triplets. But he finished the act a split second before a sheet of rain descended.

Can't you imagine the kind of action that the Covent Gardeners got in those three famous October weeks? Operas with four and five acts flashed past in two or three. Cigarette smokers, who always know to the second how many minutes it takes for the second act of "Carmen" were astonished to hear it ending 15 minutes sooner than ever - only to discover, as they leaped from their seals to head for the lobbies, that Act Three had already begun.

They say some Londoners went so far as to complain about the missing intermissions.But, if you think about intermissions for a minute, that's not such a bad idea. Take the first two acts of "Boheme," for example.

When Rodolfo has just told his friends that he and his new girl friend will join them at the Cafe Momus right away, why should we have to wait for 20, usually 25, minutes? All they have to do is walk out the door, go down a few steps, turn the corner and there they are at the cafe. No costume changes, and heaven knows the singers don't need a rest. In fact, they are all ready to go. It isn't as if they had just sung the first act of "Tristan."

True, there are dangers in this kind of thing running amuck. Just think what would happen if it were to spread to the symphonic field! Conductors would be hired, not for their artistic insights but because they tore through Beethoven's Ninth Symphony in a world-beating 18 minutes, 27 1/2 seconds, surpassing the Russian record of 19 minutes flat. Without cuts.

It might be the answer to those Bruckner and Mahler symphonies that go on and on for an hour and a half. Just move that baton twice as fast and you play the Mahler Third in 47 1/2 minutes instead of what had been thought to be its normal length of one hour and 35 minutes. Audiences could go home at 9:15 instead of 10:30. And that famous piano piece would be correctly known as the "Half-Minute Waltz."

P.S.: London can relax. The musicians have signed a new contract and opera can go back to four and five hours.