My shipping has deteriorated massively in the last couple of years. What used to be 2 day shipping as a prime member is now often as much as 10 days. That, plus all the knock-offs and bad product quality, has made me stop using Amazon for much.
anecdotally, it seems really bimodal for me. Either I can get something same day/overnight, or it'll take a week or more
They've built out small, local distribution centers that stock the more commonly ordered items. When you're in an area with one and pick the right item, it comes same or next day.