U are right some of it depends on the seller. I know on occasions I have been guilty of sending out on day 2 rather than day 1when when its been busy and there's a delay getting the part out of the car. A couple of times it was slightly longer but that's out of 200 plus items. But I also have been on the other end, waiting two weeks for something only to find seller has not even sent it out and yes, it is annoying.
BUT altho I have had the other sort, I have a dream of a postman. If he can't post something thru door and I'm not answering he either leaves a parcel on the front step under something or round the back. I find that helpful and round here its not gonna get stolen. Normally u can set your clock by him. Can always tell when he's away, postage goes up the spout.
It was much worse when I lived in London, it seemed like I would get everyone's post BUT my own. Had one guy and once he started delivering my post went from 9.30 delivery to after 11. Then one day I saw him sauntering down the road towards my house. He'd have been a good candidate for the tortoise role in a certain fable. Complained cause post was also going missing a lot and he disappeared shortly afterwards.
Having said all that tho, generally speaking I find my post pretty fast and I rarely find one doesn't turn up at all.