The problem wasn't Swift Codable, the problem was Apple's implementation of JSONEncoder / Decoder on top of Obj-C runtime, which created unnecessary round trips.
Both are problems, and it's far from fully addressed in Swift 5.9. The new Swift implementation of JSONEncoding is "only" 2-5x faster than the old one, so while it's a big improvement it's still very slow. Codable itself is also very performance-hostile, and a no-op encoder which fully walks the input but doesn't produce any output will be slower than serde producing json.