Fast-boot experiences?

Hi,

I’m working on an automotive application using RPi boards + resin.io.

Using a stock resin setup, however, the boot process takes up to 25-30 sec. Keeping the RPi always on to avoid booting delays is not an option as this drains the car’s battery.

So I was hoping to find some pointers on how to reduce the boot process of a resin device to approx 5 sec – if possible.

Thanks!