> A contended mutex is a system call […]

Because modern mutexes are so cheap (only 1 byte directly in the data structure, no heap allocation), you can do very fine-grained locking. This way, a mutex will almost never be contended. Keep in mind that a reader waiting on an empty queue or a writer waiting on a full queue will also involve syscalls.

> […] and likely stalls all the CPUs on your machine.

Huh? Where did you get this idea? Only the waiting thread will be blocked, and it won't "stall" the core, let alone the entire CPU.

By the way, if all your threads are waiting on a single mutex, then your architecture is wrong. In the equivalent case, all your actors would be waiting on one central actor as well, so you'd have the same loss of parallelism.