> RC limits itself to modifying only relevant objects, whereas GC reads all objects in a super cache-unfriendly way. Yes, an atomic read-modify-write is worse than a read, but it's not worse than a linked-list traversal of all of memory all the time.

All tracing GC algorithms scan only live memory, and they typically do so in an array-like scan (writing some bits in the object header when a pointer to that object is discovered), not in linked-list order.