That's usually not because of IO but because of dumb memory management in the JSON parser implementation. Some JSON parsers build a huge tree out of individuall

That's usually not because of IO but because of dumb memory management in the JSON parser implementation. Some JSON parsers build a huge tree out of individually allocated items for each JSON property, or in higher level languages, build an object/property tree out of the JSON structure. That's guaranteed to be slow for large trees.