Almost certainly, the bottleneck is one or both of:
The platters can’t simply spin at full speed reading a sequential stream of bytes from one and writing it to another - they periodically have to search around to different places stitching the file’s byte stream together from discontinguous chunks or reading or writing metadata. Seek latency of the platter will overshadow any tiny delays incurred because of memory or CPU delays.
The algorithm is doing something in a fashion that causes delays (e.g. reading each file individually and waiting until it can sort out if it needs to send anything for that file before starting I/O operations for the next).
Idk if you can do anything about #1 but in similar situations I’ve had good mileage preventing #2 with “tar cj /somewhere | ssh me@host ‘cat | tar xj’” (roughly speaking, you obviously may have to adjust things to make it actually work, and on very fast networks maybe it’s better to skip the -j, but that’s the rough idea).
Edit: Oh, I misread, is this local? I saw rsync and just though it was a network transfer. What kind of speeds are you getting? Does doing “tar c /original | tar x” or something like that work any faster?