Error.localizedDescription is widely used in many iOS apps.
But it's calculation needs converting Error to NSError using _swift_stdlib_bridgeErrorToNSError function that calls Error._domain computed-property through _getErrorDomainNSString<A>(_:) call.
Error._domain uses String(reflecting: type(of: self)) that internally performs multiple casts to protocols in OutputStream.swift: CustomDebugStringConvertible, CustomStringConvertible, TextOutputStreamable and CustomReflectable inside Mirror initializer. First conformance check for each pair of (class, protocol) is time-consuming because we need to scan all protocol conformance type descriptors.
That's why calculation Error._domain takes 15 ms on launch. Screenshot from Time Profilers traces is attached:
But internally, String(reflecting:) called on metatype eventually calls _typeName(:qualified:) function with qualified: true.
Full call-stack:
Error._domainString(reflecting:)_debugPrint_unlocked. Because Any.Type does not conform any of checked protocols._adHocPrint_unlocked_adHocPrint_unlocked.printTypeName
Is there any reason not to use _typeName(:qualified:) directly? It's has no performance-overhead like _debugPrint_unlocked (because of its protocol conformances checks).