If it's like iOS devices, I'd assume less than a second to interactive, potentially so fast that most people don't notice it isn't instant.
If it's like macOS, maybe 2-3 seconds to interactive.
It shuts down completely when the battery is unplugged / swapped. Very different from other Apple products. Boot time is 37 seconds according to
https://youtube.com/watch?v=GkPw6ScHyb4&t=40m38s
It is an iOS device. It's essentially an iPad Pro (with a lot of obvious other hardware) running a modified iPadOS.