"personally I use a custom alpine live-usb that is immutable and I fully shutodwn and start-up all the time... the bulk of my startup time is copying my 500mb rootfs into a tmpfs"

If rootfs is immutable why copy to tmpfs.^1 Genuine question. Not Alpine or Linux-specific. For example, maybe it feels faster running binaries from tmpfs.

Immutable directories can be mounted from compressed files e.g., squashfs, etc., on the USB media. Alternatively one can copy only the binaries that one acually needs at a given time. Then delete them from tmpfs when not in use to free up memory.

1. I do this is so I can pull out the USB stick. But I use a much smaller rootfs that fits in the kernel ramdisk.