> I wondered why a custom kernel came so late. If you want to optimize, wouldn't you start with LFS or some source-based distribution? Autonomous software updates don't seem to be a necessity anyway on such a device.

Buildroot (which they used) is made exactly for this. With buildroot, you configure your own "Distribution" and generate a single bootable image from it.

> In addition, I wonder if it would be possible to optimize the EFI/BIOS on such a device. At least on my standard Arch Linux desktop, it takes a significant amount of boot time:

Not exactly sure about raspberry pi hardware, but a lot of other embedded SoCs have a pretty minimal bootloader that runs with u-boot, which is typically very fast (at least if you set the delay it waits for user input to 0)

> wouldn't you start with LFS or some source-based distribution

You don't ever want to actually use LFS (the manual from the LFS project) in the real world as compiling GNU is far too much work. A minimalistic kernel + busybox system is much less pain. But Gentoo would not be a bad option too.

christ, you've just shown me that I need to optimize my boot loader (systemd-boot), and how great apparently my firmware is.

    > systemd-analyze
    Startup finished in 3.259s (firmware) + 35.127s (loader) + 1.823s (kernel) + 2.927s (userspace) = 43.138s