It's probably sitting in a big truck somewhere that only leaves once it's full, to minimize cost. If shipping speed is important to you, perhaps you should pony up.
You've missed the point. The experience with other retailers seems to be quick dispatch of items to the shipping service.
The article is claiming massive warehouse investments by Amazon, yet that is somewhat at odds with shipping non-Prime items slowly. It would seem somewhat strange to deliberately ship slowly to push Prime upsells since that hands things to competitors, and is only doable if you are close to a monopoly.
It is possible that trailers (not trucks) are sitting around until full but that takes up space. They also generate the tracking id before it goes into a trailer, and you usually see less than 24 hours before it arrives somewhere outside the warehouse. I doubt there is any benefit to letting trailers fill up over several days. The shippers also use regional distribution centers so there isn't a need to have a full trailer, although full pallets would make sense. However Amazon should be able to fill those in a few hours.
Here is an article from 2008 about a Newegg distribution centre. Note that several trailers a day were filled.
