First decode of complex nested JSON takes ~7 seconds vs ~0.25s with Foundation JSONDecoder

[zshannon]

When decoding a complex JSON payload with deeply nested structures, the first decode using YYJSONDecoder takes approximately 7 seconds, while Foundation's JSONDecoder completes in ~0.25 seconds. Subsequent decodes with YYJSON are fast (~1ms).

Environment:

• swift-yyjson: 0.3.0 (04748af)
• Swift: 6.1.2
• Platform: macOS 15.0, arm64

Reproduction:

The payload is a ~660KB JSON blob containing a struct with a value: JSON? field (from SwifterJSON) that contains deeply nested arbitrary JSON.

// Timing comparison
let payload: Data = ... // ~660KB JSON

// YYJSON - first decode
let t0 = Date()
let result1 = try YYJSONDecoder().decode(ElectricEventPayload.self, from: payload)
print("YYJSON: (Date().timeIntervalSince(t0))s") // ~7 seconds

// Foundation - first decode
let t1 = Date()
let result2 = try JSONDecoder().decode(ElectricEventPayload.self, from: payload)
print("Foundation: (Date().timeIntervalSince(t1))s") // ~0.25 seconds

Observations:

• The slowness only affects the first decode
• Subsequent YYJSON decodes of similar payloads are fast
• The issue may be related to recursive Decodable handling for nested JSON types
• Creating a shared decoder instance doesn't help (ruled out initialization overhead)

[mattt]

@zshannon Very interesting. Thank you for raising this issue.

My sense is that this penalty is primarily caused by Swift runtime metadata/witness table initialization. If we aren't seeing it in JSONDecoder, it could be from it already being used by system frameworks. But there's a good chance we're making unforced errors here.

To confirm, did you see this building in release mode? I've seen debug mode exaggerate this kind of cold-start overhead in other situations.

[zshannon]

I've only seen it in tests so far (incidentally the tests are Replay backed SSE streaming).