A major benefit of immutable data is worry-free sharing, and it's how you make persistent data structures fast. You simply can't afford to copy the whole world

A major benefit of immutable data is worry-free sharing, and it's how you make persistent data structures fast. You simply can't afford to copy the whole world on every update. There are some performance implications with NUMA, but there aren't serious multi-threading problems with immutable structures. Immutability means there isn't anything to race with, as long as initial construction is retired properly before other threads see anything, which is a compiler / runtime implementation problem, not an end user programmer problem.