A friend of mine is extremely fortunate to have a bit more of an 'old school' environment. They have a TV, but she doesn't let her kids use her phone. She's able to be a stay-at-home-mom, supplementing the household income with baked goods and Etsy projects and eggs from her chickens. She pays attention to her kids, not as a helicopter parent, but as a genuinely involved parent - going on walks, taking them to the library, teaching them how to interact safely with the chickens, having them cook with her, teaching them arithmetic and reading, playing with them, giving them simple chores...really making it a point to focus on early childhood education. This in turn is evident in her kids' longer attention spans, and ability to have discussions at levels in excess of their peers.
Something tells me that they will do far better than their peers on standardized tests...not because they had less screen time and spent their formative years staring at the wall instead, but because she's been an active parent and made it a point to make the most of the pre-kindergarten years.
She's an exception, sure...but the point generally stands - parents who just hand their kid an iPad and leave them alone are going to end up with kids focused on entertainment rather than exploring their world and gaining understanding, which will likely be reflected on standardized test scores to some extent.
I would also submit that one of the contributors to this problem is how basically every video game has devolved into a skinner box and dopamine dispenser. Puzzle games exist, but it's an incredibly exhaustive process to load an iPad exclusively with games that are pay-once, no-IAPs. It would be interesting to see if such a thing *could* be used as part of an experimental group, where kids who only played games that had traditional progression mechanics were compared to kids who had games that were colorful slot machines.