For example, if all objects' properties were immutable after instantiation, could this fact be relied upon to engineer higher performance in JS engines?
If I understand your question, you're asking what is one thing currently missing from JS that if added would yield a large perf gain?
I think the biggest one would be from types. The JS runtime engineers have improved perf a ton already and I imagine all of the low and medium hanging fruit is already done.
Based on the OP's comment with an example, it sounds like the question is more like "what feature/capability/etc currently in the spec could be removed to improve performance?"
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[ 5.0 ms ] story [ 17.9 ms ] threadJust curious.
I think the biggest one would be from types. The JS runtime engineers have improved perf a ton already and I imagine all of the low and medium hanging fruit is already done.