The cost of using a textual format is that floats become so slow to parse, that it’s a factor of over 14 times slower than parsing a normal integer. Even with the fastest simd algos we have right now.
rx works very well as a kind of embedded database like sqlite, but completely unstructured like JSON.
Also I'm working on an extension that makes it mutable using append-only persistent data structures with a fixed-block caching level that is actually a pretty good database.
Also note it stores decimal in a very compact encoding (two varints for base and power of 10)
That said, while this is a text format, it is also technically binary safe and could be extended with a new type tag to contain binary data if desired.
What's expensive is very relative to the use case.