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Did they really get such a performance boost (x6) over standard coroutine implementation, or was it because of the single-threaded implementation?

I’ve carefully analyzed these benchmarks.
The first improvement is achieved by narrowing responsibility scope: 
the channel becomes single-producer/single consumer, `close` and `select` support is dropped. In general, it’s a good idea to write very specialized solutions for high-performance code, but such solutions are rarely useful outside of specific use-case.

x6 boost is achieved *only* because solution became single-threaded. Note: not SP/SC, but single-threaded, producer and consumer should be executed on the same thread. No `volatile`, no barriers, no memory synchronization.