Controlling the pace

On a basic level, there are two kinds of pace. On a small scale, there's the pace of a particular paragraph. This is the pace controlled by things like the sentence length and individual word choices. On the larger scale there is story pace. This is controlled more by the balance of different types of prose sections in the larger story structure.

Let's start with the big picture

Story pacing

Direct Action vs. Summary

A key indicator is the balance of direct action prose and summary prose. Direct action tends to be at faster pace than summary even though, on a word for word basis, much less happens.

Direct action is where the reader is with a character and seeing the world through that characters eyes. Something happens, the character reacts. Something else happens, the character reacts. A continuous thread of cause and effect moving the story forward. Dialog is pretty much always direct action, and therefore will almost increase the pace of a section.

Summary sections involve the abstract description of events that the reader doesn't get to witness first hand. This is the place where characters reflect on past experiences, wonder about the future and go through events that are too long or unimportant to be shown in full. Months and years can pass in a single sentence. You can cross the sea, travel to a distant star or just sit on the bed and worry about your mortgage. Whatever is happening, the reader is not there in the moment and so it can feel slower.

Changes of direction

Changes of direction increase the pace. Each zig or zag, where new information takes the character in a new direction is like a pulse beat. If they come close together the heart beats faster, the feeling of pace is increased.

A slow, moody story may contain no abrupt changes of direction. Atmosphere built slowly over a number of seemingly straightforward events can be used to slowly bring the narrative round like a ocean liner changing heading. Inevitable change can be very powerful. Watching the ship turn, painfully slowly, never knowing if it will be enough to avoid the rocks.

A thriller may change direction like a wasp trapped in a box. Quick! To the university! Oh, no! It's a red herring! Quick! To the beach house! Oh, wait! A double cross! Quick! To the morgue! The mad-dash approach keeps the reader hooked by constantly wondering "What can possibly happen next?!" Of course, it's easy to overdo this and end up leaving the reader confused and lost.

Prose Pacing

The pace within a section of prose is controlled by the words and sentences you choose. It's possible to make scenes full of action and mayhem feel slow and tedious and moments of quiet reflection feel rushed and thin.

Description vs. Action

As a general rule, the more description and related information that a section of prose contains, the slower it will read. Long detailed descriptions of the window frames, curtains and sideboard or wordy asides about the choice of carpet all take up words that could be used to have something actually happening.

Sentence Length

It is often the case that short, sharp sentences read faster than longer sentences. This is not to say that fast sections of prose should contain exclusively short sentences nor that slower sections should only contain long sentences. It is just that a tendency towards the shorter, on average, has the effect of speeding up the section.

Word use

Fast paced writing requires simple words. Quick, easy to read words. Obvious words. Slower sections of prose can linger on big and clever words but unusual words, like opprobrium, tend to trip up a reader who is moving quickly. Tripping up can slow you down.

So which is better, fast pace or slow pace? Well - that depends on your story. It usually a good idea to carefully vary your pacing depending on the section of the story you're working on. Some stories are very faced paced throughout, others are slower with sharp rushes of action.
A story that is always slow paced throughout may become dull, but pace is relative. Even small variations in pacing can be enough to keep a leviathan of a story flowing along.