The fascination of art has to do with time. Visual art is the quickest of all serious cultural forms to make its full nature clear to the beholder. To know what is in a film you have to watch for an hour and a half or more; to read a novel can take days, weeks or months, depending how much time you have. Art – unless it is video art, or performance – takes by comparison no time at all. You can register a pictorial work by Gilbert and George – know it – in a minute, and you can take in a painting by Rembrandt in the same instant way.

It's worth dwelling on the rapidity of art. It is conventional, in the moralising rhetoric of the critic, to say Rembrandt repays a lot more time than most works of art – you can look at his pictures for a lifetime and still find new depth in them. But what if this is not the most important thing about great art? What if it's the instant effect that matters?

Leonardo da Vinci thought that. In a declamatory passage in his notebooks he argues that painting is superior to writing because a powerful or beautiful painting can be understood immediately, while you have to listen to a poem for ages before you can judge it. In other words, Leonardo's view of art is completely compatible with our culture of instant and seductive images.

The Mona Lisa proves that his idea of art as a rapid and effective form of communication fits perfectly into the modern world. Why is this painting an icon? Because its features are so graphically lucid, so easy to assimilate. And is the Mona Lisa a great work of art? I believe so.

In other words, not all masterpieces need deep study to bring out their beauty. Art really is quicker to make its impact than a symphony or a play. Recently, in the Louvre, I had a powerful encounter with Poussin's Et In Arcadia Ego. Its delicate lights and darks, the intimations of mortality that chill the landscape's dappled air, make an unforgettable impact. I felt the grandeur of this sombre pastoral in a way that will resonate in me for years to come. How long did I look at it? No more than three minutes.

So it's not always true that great art takes a long time to appreciate and instantaneous art is shallow. In fact, some of the most revered paintings can be appreciated much more quickly than video art – which has, as I've already conceded, brought narrative time into the gallery. Why is time-based art so popular? Does it seem more important because it takes up time? Personally, I agree with Leonardo da Vinci. The most magical thing in art is the instant and complete image.