You don't think the global pandemic currently in progress has anything to do with it?

It's highly variable. I've had Prime packages take anywhere from 1-5 days. Just yesterday I ordered two items on Prime, one has already been delivered and the other one might not be here until the end of the week. Both items are fulfilled by Amazon.

Given the substantial increase in online shopping prompted by the pandemic, Amazon is doing pretty well. Other retailers are doing much worse, including Walmart which has been the least reliable shipper I've bought things from.

Amazon's ability to adapt while other companies are falling on their faces is pretty impressive all things considered.

Anecdotally, I find that Amazon shipping (with prime) is always the fastest in a major urban area. My USPS and FedEx packages have often been delayed by several days, but Amazon mostly delivers on time with next day or two day shipping. Some “rarer” items have longer shipping times but I find they are still accurate.