Interesting post from Bootlin:
https://twitter.com/bootlincom/status/1382340366842654720
We started from an initial boot time of 10.5 s, and we have now a system that boots below 3s. Here are the main optimizations we implemented:
Course material for “Embedded Linux boot time optimization training” is Creative Commons licensed and can be downloaded from:
https://bootlin.com/training/boot-time/
Personally, I have taken two Bootlin training courses last year and found them to be incredibly helpful. If you have training budget, then I highly recommend Bootlin courses.