Let's kill our Darlings!

A lot of stories suffer from inconsistent or outright poor pacing. I think it's time for us to become more honest with ourselves and put our manuscripts on a strict pacing diet.

When a writer advises you to "kill your darlings", they usually don't mean that you should just go and randomly kill off one of your characters. What they mean is, simply, let go of those scenes, lines, dialogues, and characters that don't contribute to your story. You only kept them because you liked how you masterfully played with words, or because it was such a sweet moment between two characters. Or maybe because you had this scene planned from the start and now you don't want to get rid of it, because... what? Does it get customary rights?

Ask yourself the question: What's more important, the greater good of my story, or these words that I'm a bit too attached to?

Just. Cut them out of your story. Ctrl+X. Awayyy with them. If you really don't want them to be gone forever, put them in a scrap document. From there, you may be able to re-use them in a different context, or even a different story.

How to identify those Darlings

There's a few types of Darlings, from what I learned. This is just an approximate list. There are probably more types, and some overlap happens. I'm just giving examples of things that bother me the most about stories I've read (and written)!
The worst offenders in my opinion are the Repeaters. What is a Repeater? It's basically a scene, conversation or (*shudders*) monologue that just ruminates on what the reader already got to know earlier. We learn nothing new. We just hear the events from earlier again, maybe from another perspective. This doesn't move the plot forward. This may be an awkward attempt at character development. This is boring. Get rid of those, immediately.
There's also the Fanservice Fluff. In its essence, fanservice just exists to please the reader... Think of the notorious Beach Episode in animes. Fanservice Fluff could be disguised as almost anything - a cool exchange of one-liners between two characters, or a chapter-long battle scene, or a detailed trip to a car race, or the main character having a cuddle session with their love interest. The important part is that ultimately, these scenes take us out of the plot. When the whole plot and character development comes to a screeching halt for a scene to take the spotlight instead, you ruin your pacing.
Let's not forget the grave sin: Infodumping/Expositiondumping. I'm not a fan of prologues that introduce the setting. I'm not a fan of characters telling their life story to whoever the writer has sentenced to listen to it. Even two sentences of explaining the setting/world/culture/etc. to the reader may be too much. There are many clever ways to drop little bites of worldbuilding. Again, the idea is: SHOW, don't tell.
A honorable mention goes to Outspelling. This is not so much a scene as just a general habit that we should stop doing. It can be hard to spot and get rid of. Basically, this makes your reader feel stupid. That they can't read between the lines, or add 1 and 1 together. You are so busy explaining (aka TELLING) your plot and character motivations that you bore your reader with things they already understood without you shoving their nose onto them.
It should be simple: Don't explain the obvious. But in practice, we may do it without realizing. Our readers may not realize what's going on either, but they will feel patronized somehow.

Help! Where did my manuscript go??

If you followed this advice and find that not much is left of your story... Then this diet was really urgently needed. You can now focus on what is left, and the gaps between those things that hopefully became obvious after all the unnecessary baggage is gone.
Work on filling those gaps now, or check if anything that's left needs more "meat".
Or maybe you need to go back to the drawing board completely, and rethink your plot and characters from the ground up.

Please note that what is necessary and what is unnecessary for a story depends hugely on the genre.
Some readers want to see pages upon pages of fluffy interactions. They are here for that, and they would be disappointed if you cut that from your story. But if you are writing a genre where this is not the focus, you will disgruntle and bore your readers.
If you are writing for example a drama with a tragic end, some "fluffy" romance scenes may be needed to show how things used to be between the characters. They will show the reader what is being lost when the relationship starts falling apart. However, this is not really "fanservice" then, is it? That's the important part - these scenes will serve an important purpose.

A word of warning: If you start killing off the darlings, you may get triggerhappy. Be careful you don't start cutting off limbs from your story's body! You may end up with something that is very short and lacks soul. Or you may become too paranoid about everything needing deep meaning. Don't go over-analyzing every sentence.