The problem is that once threading gets involved they have to be interlocked adjustments and that's very slow due to all the cache coherency traffic. Refcounts are also branches that may or may not be well predicted. And you're filling up icache, which is valuable.
You could have a hybrid recount where you use basic integers when adjusting the ref count on the current thread and atomics to adjust the global count when you hit 0 (hybrid-rc is the crate you can use but something Swift-like where the compiler does ARC for specific values when you opt in may not be the worst). Also when the type isn’t Send then you don’t need to do atomic refcounts although the interaction with unsafe does get a like more complex.
But yeah, at this point Rust’s niche is a competitor to c/c++ and in that world implicit recounting doesn’t have a lot of traction and people favor explicit gc and “manual” resource management (RAII mechanisms like drop and destructors are ok).
