A typical car is broken down into many small, dedicated computers.
The powertrain and body control modules, assuredly running a RTOS, certainly do. The PCM for example controls when the spark plugs fire. If it did not boot instantly, the engine wouldn't run. If the BCM didn't boot, the doors wouldn't unlock nor would the key or PTS start the engine.
The user-facing infotainment crap is not necessary for the "car" part to function. You could physically remove it and the car would still be drivable.
There should be a key you can hold down which would mean "skip all enumeration and assume everything is exactly as it was the last time you booted" (with a brief warning to the user about the possible risks introduced by this)
If we're going that route, I'd like a bootloader-supplied kernel option that defaults to enabled (ideally with the GRUB/whatever menu having am option to disable it.
The powertrain and body control modules, assuredly running a RTOS, certainly do. The PCM for example controls when the spark plugs fire. If it did not boot instantly, the engine wouldn't run. If the BCM didn't boot, the doors wouldn't unlock nor would the key or PTS start the engine.
The user-facing infotainment crap is not necessary for the "car" part to function. You could physically remove it and the car would still be drivable.