‘Pacing’ is often taken to mean a story that proceeds at a fast rate. However, I would contend it doesn't just mean the speed but also includes the rhythm of your story, and I believe it’s important to take both into consideration. A story which is ‘fast-paced’ throughout may keep your readers reading, but there’s also a danger in making it too fast. I recall reading a fast-paced thriller, and literally had to stop reading at the end of one chapter. I was mentally exhausted because everything moved so quickly!
This is where ‘rhythm’ comes into play i.e. slowing down from time to time to allow your readers time to absorb the previous fast-paced scene and maybe to learn more of the character(s) reactions, thoughts, and emotions.
Looking first at the fast-paced or ‘action’ scenes: these move the story along, and shouldn’t contain distractions such as lengthy descriptions. Instead, use a few brief details to act as relevant ‘props’ for the scene, and keep the characters’ thoughts or feelings to a minimum. Rapid-fire dialogue will make a scene much stronger than long-winded conversations. Think of it as a fast ping pong game, rather than a gentle game of bowls. Also, if the outcome of an action scene is left hanging, the reader will turn the page to find out what happens next. End the scene with a revelation, a threat, or a challenge to grab the reader’s interest.
Fast-paced scenes also involve appropriate word choices, and usually short sentence and paragraph length. Cut out unnecessary words; eliminate extraneous information e.g. a character driving to a rival’s house (unless something significant happens during the drive); avoid small talk and get your characters straight to the main point of the scene.
After a fast-paced scene, you then have the opportunity to allow readers time to catch their breath and absorb what has just happened. This is when you can go into more detail about thoughts and emotions, which will bring the pace down. You can use longer sentences and include more description, but again this must be relevant and should move the story forward in some way, and not simply be a 'filler'.
The best stories are those which contain sequences that move at different speeds. Too much fast pace can exhaust the reader; too much slow pace can bore them. The trick is to keep the reader engaged by alternating the pace of your story.