I'm told that sometimes my fiction reads too fast. How can I slow it down and still keep things intense?

As a writer, you have a lot of flexibility in how swiftly or slowly passages read. Some heightened moments are most effective when slowed down. Closer attention to the details of an individual scene can create anticipation and tension. But some moments of urgency benefit from a faster pace. In the short story "Jimmy Underwater," author Julia Whitty starts with a quick pace, recounting Jimmy's near-drowning as a child:

He heard the ice cracking, the sound traveling up through the soles of his feet. Fissures shot out around him as the surface of the lake sagged. Jimmy saw that he was trapped in the center of a web of broken ice, that he was too heavy, that water lay beneath the thin, transparent surface and was lapping at its underside like a great gray tongue. He was nearly to shore but that was no help. Two thoughts came to him: that he was eleven years old and that he was about to die.

The quick succession of details speeds up the narrative, echoing Jimmy's frantic state of mind in this moment. Directing the reader's attention to the ice and the sensory details associated with it also helps to create intensity. Look what happens to this passage when the pace is slowed:

He heard the ice cracking, the sound traveling up thorough the soles of his feet. At first, it seemed like a thrill—something to leap from on this plane of glassy, flat ice. Fissures shot out around him, veins radiating out from his feet, some racing toward the shore. In the center, he was the source of power and the cause of the break. But he was too heavy and he heard the ice creak, like a great steel structure leaning in the wind, its bolts and joints scraping against one another.

In this version, I added more detail and contemplation. The focus is not so fully on the ice, as it brings in his sense of power and the image of a steel structure. The sentences are also longer and fuller, making for a very different experience. There's anticipation for what will happen to Jimmy, but not the flurried, chaotic feel of his state of mind at the time.

A fast pace can lose its luster—even when depicting the most urgent of situations—when it is so fast that the writing seems abrupt:

He heard the ice cracking. He was trapped, too heavy, and he could see the water under the thin surface. He was about to die.

This takes all the finesse out of the passage. The details are thrown at the reader one after the other and there's very little to actually experience with Jimmy. Instead of being in the moment with him, we just skim over it.

Intensity isn't necessarily tied to the length of a scene. Pay attention to which details you choose, how many you include, and how they impact the forward momentum of the specific action of your story.