I'm not sure what you're talking about then, in zigs case you don't need an exposed ABI to use existing c code?

in terms of performance it will always be ambiguous as implementations are hard to compare across languages, however they use llvm as an alternative to support architectures as they build out their native compiler. in the transition for x86_64 they were seeing up to 70% increase in compile speeds. its still ambiguous as its dependent on their implementation of llvm, though its such a large increase it is still meaningful.

but i say all this as a rust and zig advocate where i would happily choose either over the the majority of alternatives.