Surprisingly slow parsing

[mgree]

When running on the citylots JSON file, parsing takes a long, long time. Way longer than I expected---since I was told to "expect in the ballpark of 500 to 1000 megabytes per second deserialization". I suspect I'm doing something wrong.

Relatedly, lazy loading only saves time on loading, not parsing. Can we use serde_json::RawValue and similar tricks to parse but defer construction? I'm not sure every format supports this.