Which performance metric are you looking at for "faster"? Async is cooperative multitasking applied at a different level of abstraction. Much like OS level multitasking it adds overhead, and reduces performance in terms of latency. On the other hand it improves throughput by allowing better resource use.

>don't you havev to allocate that input buffer each time?

Have to? No, you could pre-allocate and reuse buffers. It is less straight forward than the buffer per thread strategy, but possible.