Benchmarks vary wildly but Haskell tends to perform in the same realm as C++, fwiw. It's GC'd, uses immutable data structures, and uses a green threading model

Benchmarks vary wildly but Haskell tends to perform in the same realm as C++, fwiw. It's GC'd, uses immutable data structures, and uses a green threading model that can be easily exploited by user code for extreme levels of parallelism with full memory safety. It's even possible to hold mutable references and share them across threads safely with STM (not a global lock). If you want fully lock-free it might be possible to prove that with extensions like Liquid Haskell, via Linear types, and is definitely easier to prove with a theorem prover than it is, generally, for C++ code. Not sure about Rust though I realize quite a lot of its moving parts have already been formalized which is super cool. There are reasons for using Rust but it's not the only game in town. And GC doesn't automatically mean pessimistic performance.