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We might first try approaching the writerly subject of "pacing" by using analogies.
I'll start with this fortuitously available photograph of the moon, by Andif..
You could say that pacing is like any 24-hour period of the sun and the moon. There's the initial drama of the dawn, which lets down slightly into the ordinary and yet ever-building rising of the sun, which hits a dramatic high point at noon, followed by the let-down of early afternoon, then the pace starts to pick up again as we move to sunset. . .
Most of us probably don't want to live in a place where there's 24-hour high-noon sunlight, or 24-hour moon-lit darkness. It would get on our nerves eventually. Never-ending sunshine might keep us too "up" for too long; never-ending night might depress us, even to suicide. We wouldn't be able to stand the sameness. We would long for a shift, a change, something different but which also smoothly connects with what came before and what will come after. . .
That's pacing. Nice work, oh great Writer in the Sky!
By the same token, most of us don't want to read something that moves at the same pace throughout. We want to go up and down and all around, with shifts in mood and the propulsion of the story, with pauses for us to relax a bit from high action or high drama, and with climaxes that really do seem like high points because of the lower, slower points that came before. We want, in other words, for stories to read like a particularly interesting 24-hour day. . .with a dawn that grabs our attention, and then an interesting morning, building to a high point of surprise and tension, and then a relaxing, followed by ever-increasing build to the last big drama of the day, and then sometimes followed by a brief winding down. . .
I also think of pacing as being akin to rhythm in music. It helps if you have natural rhythm in your dancing and your writing, but I think it's possible to learn it to some degree, mainly by paying attention to it in the work of writers you enjoy. How do they do it? Chart it out in one of their books. Try to "feel" it, the way you'd feel rhythm in music.
What do the rest of you think? Want to add anything to this? Either with analogies, instruction, or the wisdom of experience as a writer or reader? I'd particularly love to hear from you readers. What pacing do you hate, what do you love, what stops you from continuing to read, what pulls you along to read more?