When reading through a manuscript the first thing I think of in each and every scene is relevance. Is what is presented in this scene relevant and necessary to move the story forward? Of course there is always a bit of room for world-building and description to immerse the reader in the story, but if you're going to go on about something, it must be relevant. Backstory can be worked in, but it must be sprinkled, not doused, much like a strong spice that you don't want to overuse. And, of course, it must be integral to the story and what it to come. The key to backstory is to try as hard as possible not to put it in the first ten pages. You probably won't be able to resist, but trying will at least minimize it.
It is vital to start your story as close to the inciting incident as possible. Identifying the inciting incident can be difficult, and one can be tempted to think something in the backstory might be the inciting incident. Don't fall into that trap! The inciting incident is the thing/conflict that propels the character forward, forcing them into action that leads to the outcome. It sets everything in motion. Backstory does not do that. Backstory might determine why a character acts a certain way or does certain things, but it does not set everything in motion. Think of the inciting incident as the first main conflict in the story, and as that, it needs to connect with all the following conflicts and inevitably, the resolution.
To simplify it, think of the meat of your story, what it is really all about. Now that you have that, make sure your inciting incident ties into it directly. Don't start with backstory, start with what is relevant.