I am designing a custom OS for the RPi Zero W for an embedded application. The idea is to use Buildroot to strip the Linux Kernel right down to it's bones using a custom bootloader, static device nodes, etc. I'm hoping to achieve the 2-3 second boot time I've seen on some Youtube videos, which hopefully should be achievable seeing I don't need any GUI.
Anyway, to make development a bit easier I'm hoping to test the builds using QEMU or something similar. I'm wondering - how faithful is emulation of the RPi going to be? Will all devices be emulated at the hardware level or does QEMU merely emulate the ARM processor? Will the speed be comparable to a real RPi or will it be faster or slower?