yep. I built custom JSON parsers as a first solution. The problem is you can't get away from scanning at least half the document bytes on average.

With RX and other truly random-access formats you could even optimize to the point of not even fetching the whole document. You could grab chunks from a remote server using HTTP range requests and cache locally in fixed-width blocks.

With JSON you must start at the front and read byte-by-byte till you find all the data you're looking for. Smart parsers can help a lot to reduce heap allocations, but you can't skip the state machine scan.