Pacing if

The words we put onto the page create rhythms, and those rhythms control the way the reader reads your story.  They also play a part in engaging the reader in our story.  In some places we need to slow down the narrative, and in some to speed it up.

If you've just ended a chapter with a cliffhanger after a fast chase, your readers need to catch their breath before the next pursuit begins. If a detective has just risked being shot at and killed to obtain some vital information, you'll want to slow down the action while she examines that information and works out what it means.

We slow the pace to expand the emotional impact of a scene.  Characters talking about something they're emotionally invested in will want to take more time over examining a situation.  They may become introspective and think of past history, and you may want to slow the pace by adding a flashback.

Sometimes we want to pick up the pace.  Action scenes are the obvious places.  Anger and conflict are best conveyed through fast pacing.  People who argue talk fast, and interrupt each other often.  Snappy dialogue picks up the pace, and gives the illusion of something happening.

To speed the pace, avoid long sentences.  And avoid breaking up sentences with several commas.  I often have to edit my original thoughts this way to break a long sentence into two. Short paragraphs  speed the pace.  I've got many books by writers who have paragraphs of one line.  Focus on the essential detail, not the bigger picture.  And when we're in the middle of being shot at we have no time for introspection.  We just need to get away from this madman.

Pacing it is about balancing the fast and slow parts of your story.  And there's no formula for getting it right.  It depends on your individual story, and on the genre you're writing in. For a literary book a long, lyrical style might be right.  For a thriller it most certainly won't be.  Knowing the right pace is an instinctive thing, something which is helped by reading a lot of other readers' work in the genre you're writing in.  And in the end, getting it right is down to your gut feel.  Do what feels right for your story.