The simple model for scp and rsync (it's likely more complex in rsync): for loop over all files. for each file, determine its metadata with fstat, then fopen and copy bytes in chunks until done. Proceed to next iteration.

I don't know what rsync does on top of that (pipelining could mean many different things), but my empirical experience is that copying 1 1 TB file is far faster than copying 1 billion 1k files (both sum to ~1 TB), and that load balancing/partitioning/parallelizing the tool when copying large numbers of small files leads to significant speedups, likely because the per-file overhead is hidden by the parallelism (in addition to dealing with individual copies stalling due to TCP or whatever else).

I guess the question is whether rsync is using multiple threads or otherwise accessing the filesystem in parallel, which I do not think it does, while tools like rclone, kopia, and aws sync all take advantage of parallelism (multiple ongoing file lookups and copies).

I’m not sure why, but just like with scp, I’ve achieved significant speeds ups by tarring the directory first (optionally compressing it), transferring and then decompressing. Maybe because it makes the tar and submit, and the receive, untar/uncompress, happen on different threads?