Pacing is something I personally struggle a lot with. By virtue of how I write (with little to no preparation), it can be hard to figure out how fast scenes should be. That said, there are several things I try to keep in mind when I’m thinking about how fast the words should feel on the page.
Now, obviously time span is a loose term in writing. I can literally say “thousands of years later”, and in the context of that sentence alone, that amount of time has passed. So, when we talk about pacing and how long a scene really takes to occur, we’re talking about two completely different things. A short story can be a single instant in time or millions of years long, but it will have (almost) no impact on how fast paced the story feels because that part relies entirely on wording.
I’m going to bring up something that’s pretty obvious here. Obviously, going into more detail, using more words, and making the sentences longer in general would naturally make the story feel slower. If you use lengthy sentences, your reader will be forced to slow down, and it will make the entire progression of the story slow to a certain degree, and this specific sentence could be used as a good example.
On the other hand, short sentences change that. Small, clipped words makes reading easy. Don’t make it complicated. The simpler, the better. That way, the reader can go through it quickly.
I like to make lots of paragraph breaks, too.
It may not seem like much, but if you deliberately elongate or shorten sentences, it will naturally have a huge impact on how it is read. This isn’t important all the time, but in a scene that you want to increase a reader’s heart rate, or make a reader more emotional, it is another tool to be used.
Another trick is to vary the length and type of actions in the story. In an action sequence, a lot of the things happening are important. A broken arm will hurt, and it happens quick. But you still have to worry about not breaking your everything else, so the tension rises. In a slower piece, a character might wash the dishes, then look outside and reminisce about the past, then pet their dog, etc. None of those are very big or important actions, so including them will slow down the tempo of the thing being read.
This is also a huge reason why “info dumping” on chapter one is such a bad idea. Telling us about the history of the characters or the place makes the piece feel so much longer than it really is, and since the reader isn’t hooked yet, they’ll just put the book down. That’s why the forty page introduction to Fellowship of the Ring feels like an eternity–there is literally nothing happening in the book.
In general, if the scene is not one that is meant to feel particularly slow or fast, the safest best is to vary sentence structure. Don’t get your reader invested into more than you need them to be, and just relax. I think most writers will naturally find the ‘natural’ story pace given practice.