[WhitneyLand]

Thank you for the clarification. I think we are saying the same thing.

In principle, it's always a certainty that JS can be written as performant as Dart, never just a possibility. The possibility part only comes in with respect how optimally the JS code is written.

The interesting part here is I think is how often does this happen? How often can significant improvements be made that would be impractical, or even unlikely for a developer to match?

[esrauch]

I suspect that depends on your definition of significant. I suspect any legible code can always get a few percent speedup by doing things that would make it unreadable and unmaintainable.