But using an existing parser that happens to be written in C could definitely be sensible.

Yes, we do so for more complex formats like YAML and XML. However, for JSON, reimplementing is worth it for binary portability.

That sounds like a pretty awful idea. If you're using a C library for parsing JSON then you must add an additional C to Nim/Crystal conversion step that requires additional RAM and CPU time.

Nim compiles to C, there is no cost in calling C.

That's not how people use C libraries.

thats not how it works

it still compares algorithm implementation, not the language speed.