As I understand it, each iOS application is sort of like its own 3D plane within a larger environment, hence why the launcher shows up without any lag.

I hope someone can do the work of pasting the original Aqua framework overview that’s probably still hiding somewhere on the Apple website. The manner in which the combination of OpenGL (Metal?) and PDF work to render UI and elements on OS X and iOS is really quite remarkable. I think even now, 20 years later, there isn’t anything comparable being done by Android/Linux or Windows. I would love to be proven wrong, however (I haven’t followed this closely for the past few years).