The copy is made by the runtime, which obviously does use atomics underneath, but you don't ever use these yourself.

The workers are threads. The separate memory is the concurrency model, but it's still single process, multiple threads.

The "faster than" is on average, and both in my own benchmarks and independent benchmarks you can find examples of algorithms where it is slower, but they tend to be the exception. "substantially" slower I haven't found yet.

https://kostya.github.io/LangArena/#/top_tab

That site is just benchmarking how well an AI can implement things, it has nothing to do with how "fast" the language is.