In addition to what others said, even if counts have to be updated, that need not always use atomic operations. See https://dl.acm.org/doi/10.1145/3243176.3243

In addition to what others said, even if counts have to be updated, that need not always use atomic operations. See https://dl.acm.org/doi/10.1145/3243176.3243195 https://dl.acm.org/doi/10.1145/3243176.3243195: “BRC is based on the observation that most objects are only accessed by a single thread, which allows most RC operations to be performed non-atomically. BRC leverages this by biasing each object towards a specific thread, and keeping two counters for each object --- one updated by the owner thread and another updated by the other threads. This allows the owner thread to perform RC operations non-atomically, while the other threads update the second counter atomically.“ (I don’t know whether Swift uses this at the moment)