“In the end, we will have a RPI that boots from power-up to Linux shell in 1.75 seconds, power-up to Qt (QML) application in 2.82 seconds.”
Strip the bootloader stages, the kernel and the init system down to what the device needs and a Pi flies. Most of a normal boot is work nobody asked for. The hard part is Broadcom’s closed GPU firmware, which ships as binaries you can only reverse engineer. The slowest piece of the stack is the one you aren’t allowed to see.