Commercial fiction needs to be overwhelmingly moment-to-moment, with just enough narration and backstory to add texture and to maintain clarity. The temptation is always to provide too much, which creates a great excuse for readers to put the story down.
For some, pacing comes naturally. I've noticed that people who gossip and tell jokes seem to just know when to drop in information that becomes more important, to slip in asides that put people off guard, and to add commentary and reflection that directs or redirects the audience.
Of course naturally here doesn't necessarily mean genetics. Culture plays a part. One of the most magical occurrences in my life was when a roommate woke me up in the middle of the night and spirited me off to the backwoods of Virginia for storytelling and moonshine. Many (U.S.) Southerners are brought up in an environment of storytelling, and they seem to weave tales effortlessly.
For the rest of us, it may be work. One effective method is to mark all the digressions, descriptions, explanations, and reminiscences in your text and eliminate them all. If the story still makes sense without them, you may be done. Or you may add back in a few to season the mix.
More likely, you will find that some of them are essential to understanding the story. Add these back in. Ideally, a reader tells you this. (It is far too easy for the writers to see every bit of backstory and narration as vital.) When these pieces are resurrected consider two things:
First, do you need every word? Less is more here. Cut any backstory you need to the absolute minimum needed for clarity and texture. Avoid the temptation to keep it all.
Second, does this piece occur as late as possible in the text? The longer the readers wait for revelations and explanations, the more likely they'll seize on them. Put the material in late enough in the work, and there will be no sense of it slowing the story at all. The words will be snapped up and gobbled down.
I'll add one more proviso: The need for, and tolerance of, these relatively static elements depends upon the genre. Some readers look for more sensual elements and a more stately pace. Rushing them along is a mistake. So read widely in the genre you're writing in. And, when in doubt, make your story slightly more spare in narration and backstory than your favorite works.