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If a language can produce the same machine code (including dynamic patching) as a second language, that second language can at best only equal the runtime characteristics of the first. The development and maintenance costs can be very different.

I am not certain if C (apart from inline assembly) can generate all possible machine code, and some translations can be fragile (autovectorization, e.g.).

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Well indeed, which is why I don't really like talking about the "speed" of a language at all. What really matters is the characteristics of the idiomatic usage.

I would guess that rate of benefit relative to divergence from idiomatic purity might also be significant. If a little clutter gains a 50% improvement with language A but only a 10% improvement with language B, language B having a 15% advantage under idiomatic expression may not be that significant.

But, yes, terms like "the speed of C" do not seem especially useful.

Even "equal effort" ignores the complexity of the effort-to-benefit function as well as maintainability issues and other considerations like finding good programmers who want to use the language.

Reality seems to be very complex.

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