Then, if you don't store contents of entire screen in memory, which simple math says you can't, I was partially wrong(depending on if you don't count buffer in display as framebuffer) when interpreted "shadow copy" as backbuffer.
EDIT: Wait. Do you have framebuffer at all? Because from sounds of it, you might not even have it at all. If you don't store entire frame in RAM, then you don't have framebuffer, not just backbuffer.
The M1 is fast enough that ffmpeg can re-encode raw video footage from a high end camera (talking file sizes in the 10s of gigabyte range) an order of magnitude faster than realtime.
Depending on codec and settings, this might be super fast and super slow.