I'd just like to point out that it's still engineering to re-implement a library into your code (it's a bit of a sore spot with me... I've been told to "just use a library it shouldn't take any time at all" in the past, and apparently I'm still not over it!)
But regardless, it's still a non-zero amount of work, and in my experience choosing a mutual format isn't always a simple meeting...
Not to mention that if JSON is still needed elsewhere in your system (either internally, or externally with others) all you've done is add complexity to your application and spend developer time all for some savings on parsing speed.
But you already accepted the cost of this by playing this game - this particular library is in C, which might be an effort to integrate with a different programming language.
On the other hand, there are CBOR or MsgPack libraries that are fast and native for many languages.
But regardless, it's still a non-zero amount of work, and in my experience choosing a mutual format isn't always a simple meeting...
Not to mention that if JSON is still needed elsewhere in your system (either internally, or externally with others) all you've done is add complexity to your application and spend developer time all for some savings on parsing speed.