I haven’t done a test specifically from 0-50%. But given the wattage difference between Apple 2.4A and USB PD is highest at those lower battery levels I would say most, if not all, of the 15 minute difference will be in that window. They boast about their 0-50% charge times cause that is where PD has really sped things up.
Apple 2.4A was made to support the iPad, which needed more juice. Apple added it to the next gen iPhone cause why not. And then they could see 12W chargers to some iPhone owners. It has remained on all model iPhones since then.
Adopting USB PD as a member of the USB-IF also makes sense. It allowed them to keep up with Quick Charge 3.0 and the larger phones/batteries needed faster charging.
The reason they kept Lightning and didn’t go USB-C is two fold.
One, in 2012 they did a change from 30-pin dock to Lightning. That was a big PR hit for them. A lot of pissed off customers having problems upgrading phones and losing old dongles/cables. They weren’t looking forward to doing it again in 2015-2016. That would impact many of the same customers, some who may end up with three model iPhones using three different connectors. So they added USB PD to Lightning and that gave them the big advantage most users wanted. Lightning was already orientation agnostic, like USB-C. The only remaining advantage was data transfer rates, which most users didn’t care about. It isn’t until now that device cross compatibility is a thing for more than early adopters. And Apple is close enough to going totally wireless that it isn’t worth the trouble of a connector change.
Two, Lightning is Apple licensed tech while USB-C is open standard. Apple likes having end-to-end control. Whatever one may think of that, it is a big part of Apple’s customer satisfaction success with their devices. It also meant a steady income stream,. Both from selling their own accessories and from licensing fees for third parties making Lightning products. iPhone profits have been declining for years, and it was the single biggest part of the company. Losing the Lightning income became more problematic each year and new iPhone release. To the point where they had to double down on the decision.
Android phones including fast chargers mostly do it as a pro against the iPhone. It isn’t because those companies like their customers better. It is one more thing they can say they do better than Apple. Apple doesn’t have that problem. If you want an iPhone then you’ll take what you are given. If you don’t want an iPhone include a fast charger isn’t going to change your mind, you have other reasons. They only added the fast charger to the premium level iPhones this last round because those customers expect the full package for the premium price.