If you allocate and free small memory chunks at high frequency in a non-GC language, then it's quite likely that similar code in a GC'ed language will come out faster. The point of manual memory management isn't that it is "automatically" faster than a GC, but that it gives you a lot of control to reduce the allocation frequency (e.g. by grouping many small items into few large allocations, or moving allocations out of the hot code path).
Also, async itself doesn't magically make code run faster (on the contrary), it just lets you (ideally) do something else instead of waiting for a blocking operation to complete, or at least give you the illusion of sequential control flow (unlike nested callbacks).
