This is GRUB’s final warning before you dig too deep in the OS list. Never hold ⬇️ for more than 45 minutes. If you do, make sure you have punch tape with a bootloader available or you’ll have to manually enter machine code instructions to get your computer back up.
You are right, QR codes are very easy to decode if you have them raw, even the C64 should do it in a few seconds, maybe a minute for one of those 22 giant ones. The hard part is image processing when decoding a camera picture - and that can be done on the C64 too if it has enough time and some external memory (or disks for virtual memory). People have even emulated a 32-bit RISC processor on the poor thing, and made it boot Linux.