alt Hacker News> I guess the question is whether rsync is using multiple threads or otherwise accessing the filesystem in parallel
No, that is not the question. Even Wikipedia explains that rsync is single-threaded. And even if it was multithreaded "or otherwise" used concurent file IO:
The question is whether rsync _transmission_ is pipelined or not, meaning: Does it wait for 1 file to be transferred and acknowledged before sending the data of the next?
Somebody has to go check that.
If yes: Then parallel filesystem access won't matter, because a network roundtrip has brutally higher latency than reading data sequentially of an SSD.
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The filesystem access and general threading is the question because transmission is pipelined and not a thing "somebody has to go check". You just quoted the documentation for it.
The dead time isn't waiting for network trips between files, it's parts of the program that sometimes can't keep up with the network.
Note that rsync on many small files is slow even within the same machine (across two physical devices), suggesting that the network roundtrip latency is not the major contributor.