Maybe an agent said your pacing is off. Or an editor, or a reader did, and you're not sure exactly what they're talking about, or more importantly how to fix it.
Pacing is something that is difficult to detect in your own writing. It requires an ability to look at your own work from a psychological perspective that's far away, which is difficult to do when you are literally sitting in it--kind of like detecting your own B.O. Happily, other people will point it out for you if they are kind, or at least, really blunt.
Although sometimes people sense pacing is off but can't quite articulate it as the reason--they might just have the sense that "the middle lagged" or "the ending felt rushed" rather than knowing to pinpoint the word exactly: pacing.
If someone tells you the pacing in your book is off, that it's too slow, in particular, the first thing to do is look at your word count. If you are over 100,000 words, in any genre, I think you want to take pacing very very very seriously. Every book that I have beta read/ edited that was over 100k could have been a lot tighter and all had pacing issues. There are good books that are legitimately over 100k words, but there's also a very good chance that your 100k+ book doesn't need to be that long, and if you're book is really long, pacing is a bit harder to sustain. Sometimes length is justified--when I saw the run-time for Avengers Infinity War I assumed it would just be wallowing around it it's own wealth, but that movie is super-tightly plotted and well-paced. Which is why people say "that didn't feel like 3 hours."
Diagnostics: what does "your pacing is too slow" even mean?
Problem: There are too many segments where nothing is happening. Sometimes something appears to be happening when actually nothing is happening. I have a method for telling when nothing is actually happening that I will get into a bit. My sense is that this often happens for scenes that purely exist for characterization's sake, for the sake of world building, or for the sake of looking at your own pretty words.
Solution: This is tedious, but it works (and I have written about this here). You get an index card for every single scene. Draw a line down the middle. On the left, write (briefly) what happened in this scene that actually moved the plot forward. On the right, write the auxiliary stuff that was also in the scene that has to be in the book somewhere, but doesn't actually move the plot forward. A plot point (on the left) could be that Frodo announces to the Council that he will take the ring into Mordor. The fact that he does this somewhat reluctantly is characterization--stuff that goes on the right. Even things that are almost entirely character driven still have plots. It's hard for any type of writing that requires a lot of world building because that stuff just takes of so much space. But you know when you are reading something and you get to a huge chunk of description or history--an infodump? That's what happens when there's nothing on the left side of the card, and all world building on the right side. Another thing that might be auxiliary--even though it is really important--is to show an important relationship. That X and Y don't get along.
You might do this exercise and discover that there are a lot of cards that have nothing on the left. (If, for example, we had one scene where Frodo agrees to take the ring to Mordor, then a separate scene that consists solely of him worrying about this decision). Any true plot point can be written in a scene in such a way that you can fold in all that auxiliary stuff. A scene where a CEO sacks every single person in the company (plot) can be combined with the fact that Becky and Miranda are besties (demonstrating relationship), the fact that the CEO is a crazed visionary who thinks technology will destroy the world (characterization), or the fact that this is a company that manufactures fax machines in 1995 in Detroit (world building). Ok, not very interesting world building-- say it is a company where they illegally manufacture pencils, because writing is outlawed in the future. You don't need a big chunk about the legality of pencils. It can be interspersed throughout the scene, a scene which is moving forward. When I'm editing someone else's stuff, the most dominant problem that contributes to pacing that is too slow is too many scenes where technically nothing is happening.
Problem: Too many examples of the same thing. Say you have one card where Becky's terrible husband does something terrible, and she forgives him. Then you have another. And another. We get it. If you did it right, we got it the first time. The best example I can think of this is when I tried to read Les Miserables. There's a scene (it might even be multiple chapters) about a a bishop (Monseigneur Bienvenu, if I remember correctly) doing something really kind. Then another scene about him doing something kind. Then a third. Point being, that this is the religious man who does something incredibly kind for Jean Val Jean in a moment when he could have punished him.
Solution: I would argue that you could jump straight to the scene where Bienvenu does the thing relevant to the plot--helping Val Jean--because otherwise a scene where he is just helping some random person serves only to characterize him, and not to move the story forward. (Les Mis is a great story. It is nine million pages and ain't nobody got time for that. My favorite musical though.). If there is one scene of Becky's husband abusing her and her staying with him, and then another scene where that happens, there needs to be something substantially different and/ or meaningful about that second scene in order to justify its existence. The second instance could be combined with some other plot point. You might think "piling more on will make this more intense." It doesn't--people want to move forward, not down. (NB: Be aware that if cyclical behavior is part of your plot (a woman who keeps going back to her abusive husband) the plot will feel meandering unless you are able to somehow instill a sense of forward inertia.)
Problem: Too much physical space has been spent on the page for the amount of things occurring. You know when a bad Saturday Night Life sketch overstays its welcome? Don't do that. (Go back and watch Key and Peele instead: these guys get right what SNL still can't. Not only do the sketches end at a reasonable amount of time, but they often end on a weird, out-there funny hook rather than a slow descent towards "Okay, I'm going to run to the bathroom while this winds down..")
You write dialogue that goes on and on. Cut out the sentences that don't actually move the conversation forward. Cut out the stuff that repeats. For the love of god, cut the "As you know Bob" speeches. Unless you are really, really good at dialogue, use it sparingly and efficiently.
A relatively small plot point is dragged out. This is kind of hard to tell on your own, in part, because it is related to the pace of things that come before this. If you're entire book is written at a fast clip, like an airplane thriller, and then you get to a part where someone gets in her car and takes a phone call AND THIS TAKES FOUR PAGES people are going to pull their hair out. You need to keep reasonably true to the rules of pace that you have set elsewhere in the book. If it's a small plot point, it doesn't need tons of space. And the fact that it doesn't take much space doesn't mean it isn't important. I would rather have one tiny chocolate truffle packed with flavor than a large cake with that same amount of chocolate in it. If you have trouble telling how long a scene should be, figure out some sort of score: a combination of how important a scene is (1-10) and how complicated it is (1-10). The higher the number, the more space it gets. Every line is real estate, and real estate is precious.
A scene is physically or logistically complicated. This is hard. Really hard. Because you don't want to be so scant as to be confusing, nor do you want a blow by blow of every little thing. I think this happens most when writing action scenes. Action scenes are hard to write without being confusing. People either say too much (way too much detail, like four paragraphs describing in detail how someone falls to the ground) or not enough (reader gets confused and doesn't know where things are physically or what has happened in the literal sense). Think about size. If you're writing the major bout between Rocky and Apollo Creed, which the entire movie has been building to, you can do the blow-by-blow. If you are, however, writing about an entire battle with thousands of people, you can't really do blow-by-blow. If you've read the original Lord of the Rings books, there's tons of battle scenes with lines like "Aragorn slew an orc." For a scene that has a lot of action, but is supposed to be suspenseful. I'm deeply flattered that several people told me I write action sequences well. If I had to guess, this is how I would say I do it: 1. You increasingly build the tension as you get close to the big action scene, which then justifies the amount of space it takes up. But the pace must be at a fast clip. 2. Literally nothing can be confusing. No ambiguous language. Read the text for unintended double entendre. Pronouns and you're not sure who they refer to. Passive voice. 3. View the scene in your head as if you were watching a movie. Then write what you would want the reader to see from the camera's perspective. "Cinematic" writing works really well because we've all grown up watching TV and movies. I like to think of actions scenes from the perspective of a director: what do I have to show so that this makes sense to the viewer? Related to this...
what about pacing that is "too fast"?
Problem: Reader is confused or feels cheated. I think the former happens with action scenes when it isn't clear what has happened. The latter is when the reader feels emotionally cheated--this is often because of a mismatch between set-up and pay-off. If you've spent an hour of movie time hyping up the fight between Rocky and Apollo Creed, and the fight is Apollo punching Rocky twice before Rocky falls unconscious, people are going to be annoyed. This lack of pay-off might be the point in some particular cases, but this would still violate expectations. Sometimes this is done well in meta-fiction. An excellent example of this is in the movie remake of 21 Jumpstreet. The two cops have several instances of small collisions (a car hitting another car) where they then cringe, waiting for the huge explosion that would normally occur in a movie even though it likely wouldn't have in real life. They comment on how weird this is. Later on, something does inappropriately and unexpectedly explode, and it's really funny. But more commonly, this is something unintentional, where the writer wanted the reader to feel the pay-off, but the pay-off wasn't full enough. A good example of this is Helen Simonson's The Summer Before the War. She spends the entire novel building the potential of this couple, you're really rooting for them, and you are just waiting for that moment for them to finally declare their love/ be together. . . but it just sort of happens. Like, "oh, we're together now." Especially in context with some other things that happen at the end of that novel, you needed a big emotional pay-off in their finally getting together. This is also the perennial problem of the "will they won't they" romance. If they get together it often isn't satisfying for some reason--the reason for this is the amount of tension put into the equivocating far outweighs the emotional pay-off of them actually being together. (I actually don't think there's a solution to the "will they won't they" problem-- I think people like seeing the perpetual tension.)