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?"

For example, if all objects' properties were immutable after instantiation, could this fact be relied upon to engineer higher performance in JS engines?

Just curious.