how can one be sure that this 20% gap is a guaranteed design advantage
It's no guarantee, but in my experience, Zig is a language that lends itself very well to composability. I imagine that instead of adding features to stdlib, anyone who needs "extra features" in their threading system will find it quite easy to implement their own as a library outside of stdlib that can easily be swapped in and out in A/B tests for the library consumer. If some feature or another is REALLY wanted in the stdlib for some reason it would probably be easy to drop it behind a comptime-flag as part of a type constructor function, which won't compromise (runtime) performance, teeny tiny hit to comptime performance.
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It's no guarantee, but in my experience, Zig is a language that lends itself very well to composability. I imagine that instead of adding features to stdlib, anyone who needs "extra features" in their threading system will find it quite easy to implement their own as a library outside of stdlib that can easily be swapped in and out in A/B tests for the library consumer. If some feature or another is REALLY wanted in the stdlib for some reason it would probably be easy to drop it behind a comptime-flag as part of a type constructor function, which won't compromise (runtime) performance, teeny tiny hit to comptime performance.