TWO BIG shows have been making art headlines of late. First, there was the new Turbine Hall commission. The Belgian-born artist Carsten Höller installed five fairground slides to send visitors shooting through the spaces of Tate Modern. Who said Belgians were boring? As gallery-goers canon down this cultural Cresta Run, vertebrae bumping over stainless steel ridges, eyes blinking frantically at bright flashing lights, the Turbine Hall echoes with whinnying shouts.

Now, today we have the arrival of another artistic phenomenon. The National Gallery opens its most important show in decades. Velázquez, the “painter of painters”, comes to Britain. London plays host to some of the finest pictures in the world. “So what do you think of it?” I asked a thoughtful art historian as we emerged back out into the evening after a preview of this quite simply superlative show. She looked at me with eyes that were suspiciously shiny. “I feel all shaky,” was all she could say.

It was funny, because that was exactly what someone had said a couple of days earlier as they helter-skeltered out of one of Höller’s slides.

Höller, if you take the trouble to read up, sets out to probe the nature of human perception, to elicit fear and delight and test the parameters of reality. He also pays token homage to environmental concerns. Miuccia Prada, apparently, travels daily down one of his eco-efficient chutes from her office penthouse to her chauffeur-driven car (though I’m not sure it’s all that energy-saving unless he also constructs the catapult to hurl her back up).

But how much of this do you hang about to find out? Of course, the experience is fantastic fun. I cycled back home smiling. But if slides, as Höller wishes, were part of our everyday life, I suspect we would simply get used to them: like those nutters who try to break records and spent successions of perfectly miserable months living in rollercoaster cars.

Velázquez also probes our perceptions and tests the nature of truth. But he can’t offer instant gratification (or eco-theories for that matter — though he does save on the lighting and seems very fond of dogs). You have to take this show slowly, let the pictures explain the developing talent, feel a slow dawning reverence for a percipience so subtle that, only with time, does it infiltrate. This is a painter whose power has gradually deepened over the centuries. His sensitivities have sunk down into the human soul. To see the pictures again is to stir sediments up.

This is so far from that quick-fix fairground thrill which puts our culture on a slippery slope. The contemporary art scene has been a lot about sensationalism, and after the adrenalin rush things can seem a bit flat. Galleries — all competing to present the next big experience, to be more impressive, more blatant, more instantly entertaining — risk losing touch with something much more profound.

It takes patience to appreciate a painting. Try spending as much time with one Velázquez painting as you do queueing for a go on a Tate Modern slide. It is only through a slow process of perceptual discovery, through the gradual sifting of responses, that the meanings and passions that seeped into the production of an art work will start to communicate. If we expect instant emotional access, we will just end up flicking, by remote, past entire passages of art history.

Museums such as Tate Modern have helped to nurture a new and sophisticated audience. They now need to speak to a developing and maturing taste. Art galleries are great institutes of education and debate. Directors must approach their programmes with rigour and intelligence. Curators must choose art works that sustain a high level of exposure. Otherwise museums will end up only by teaching us how to appreciate art less.

• SPEED is the essence in the contemporary — not least when you have a soaring reputation to capitalise on. At the racy Frieze Art Fair last weekend, Jake and Dinos Chapman were getting rich quick by offering to paint your portrait in 20 minutes at £4,500 a pop. Two brothers, between them painting three portraits an hour, eight hours a day for four days — long pause while I do my painfully slow sums — puts £432,000 in the till (unless of course someone treated them to a £13,500 lunch). That’s obviously what the fun of the art fair is all about.

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