Backstory is a tricky thing - without it, the reader is lost and can't make sense of current events. Too much of it and the pacing stalls. The reader might even put the book down. So, where's the balance? I'm still figuring that out myself, but one thing I'm sure of (at least for Young Adult) is that backstory does not belong in the first chapter.
I'm the type of writer who thinks a lot about a new story, but doesn't start writing anything down (other than a general story arc) until chapter 1. And what I've learned about my technique, is that chapter 1 should pretty much be deleted from the manuscript. But that's okay. Writing that chapter wasn't a waste - I wrote it for myself, to get my bearings in a new world and to solidify the characters and the voice.
Once I've deleted chapter 1, I go through chapter 2 and highlight any backstory. I analyze each line to determine whether or not the reader would still understand what's going on if the backstory was removed and if so, it gets cut. Then, I create a new document to put that cut backstory in for possible future reintegration. In the end, there might be one or two lines of backstory in chapter 2 (which now becomes chapter 1), but there won't be any flashbacks, or summaries of past events.
Also, if I get feedback from CPs or Betas about a slow chapter, I use the same method of highlighting the backstory to see if that might be the problem - if I haven't made the reader care about the backstory, then it will slow everything down and it really doesn't belong in the MS.
Here is a super helpful blog post on How to Write Backstory without Putting the Reader to Sleep
How do you deal with backstory in your writing?