Interesting. I given up using Raspberry Pi 4 on a project because their crap proprietary bootloader took something like 4 seconds to start the kernel. Maybe it was faster on Pi 3. I ended up buying board with Allwinner that works with U-Boot.
That's interesting. I was thinking of going down the same route. Which Allwinner board? And how fast did it boot?
I went with Orange Pi 3. I would need measure exactly, but with U-Boot compiled without unnecessary things (USB etc.) it starts loading kernel pretty much immediately. In total boot to my graphical application which uses KMS/OpenGL was something around 2 seconds.