We'd be happy to address specific problems on the mailing list. I believe it's a known issue that the Rust compiler is making really heavy use of rapid allocation/freeing cycles, and would benefit from linking a performance-oriented malloc replacement. Doing so is inherently a tradeoff between many factors including performance, memory overhead, safety against erroneous usage by programs, etc.

One statement in your post, which some readers pointed out was apparently added later, "Others have suggested that the performance problems in musl go deeper than that and that there are fundamental issues with threading in musl, potentially making it unsuitable for my use case," seems wrong unless they just meant that the malloc implementation is not thread-caching/thread-local-arena-based. The threads implementation in musl is the only one I'm aware of that doesn't still have significant bugs in some of the synchronization primitives or in cancellation. It's missing a few optional and somewhat obscure features like priority-ceiling mutexes, and Linux doesn't even admit a fully correct implementation in some regards like interaction of thread priorities with some synchronization primitives, but all the basic functionality is there and was written with extreme attention to correctness, and musl aims to be a very good choice in situations where this matters.