[Second review] SE-0395: Observability

[stephencelis]

@Philippe_Hausler Are you and the team aware of any performance issues in the current implementation?

After using things for a few weeks and migrating some model code from Combine and ObservableObject to @Observable I decided to benchmark both approaches and was surprised to see the @Observable version was about 10x slower, and spent 10% of its time in access(_ keyPath:) via _swift_getKeyPath(pattern:arguments:).

I know @Joe_Groff has mentioned in the past that there are still performance wins to be made with key paths:

Runtime performance cost of key paths

Yeah, the runtime implementation has not yet been looked at with any eye for performance, and there are also relatively straightforward optimizations for converting a keypath to a getter function without involving the key path object at all that have not yet been implemented.

I'm curious if this performance is to be expected long term, if improvements are in the pipeline, or if it'd be helpful to dive deeper into the problem.