I guess you could actually use the encoding as an immutable in-memory representation as well, even for collections, but that seems both like a hyper-optimization and pretty fringe/limited

yes this is exactly what I'm already doing! and have found empirically that yes, it does improve performance significantly.

size-delimited is simpler/easier to parse

I want to avoid parsing it!

Okay, a compromise: If you where hypothetically gonna specify a length delimited version,
as I'll need to actually make this to test whether it is faster or not, what would it look like?