Two small changes make bson-kotlinx decoding 10 to 20% faster, and about 2x faster when kotlinx-serialization-json is on the classpath.

• AbstractBsonDecoder rebuilds its element metadata for every structure.
• The JsonBson*Decoder classes build a full Json instance for every structure, even when no JsonElement is decoded.

The JSON case matters most: kotlinx-serialization-json is an optional dependency, but applications often have it through other libraries, and its mere presence selects these decoders.

I will provide a PR soon.