What would be an ideal (or more appropriate) speed?

Well it would depend on the specifics of the JSON file but eyeballing the stats at https://github.com/miloyip/nativejson-benchmark/tree/master seems to indicate that even on a 2015 MacBook the parsing proceeds using e.g. Configuru parser at several megabytes per second.

simdjson can parse JSON files at ~2.5-3GB/s: https://github.com/simdjson/simdjson