[olliej]

On apple hardware, uncontended refcounting (swift or objc) has the same perf as non-atomic refcounting. The cost exists, but it isn't terrible, once there's contention between threads the perf drops through the floor. The real killer is there are a bunch of places where the swift evaluation model means they're forced to ref churn, which comes up 100% typical workloads like the million triangle objects in my swift raytracer, all being hit by numerous threads :D