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Local rsync disk to disk slow

I am copying a large directory tree from one internal disk to another USB 3 connected disk on the same computer with this rsync line:

rsync -ahPvzHA --delete /d/Images /q/

This copies files at a rate of 20MB/s. If I copy the same tree with Windows Explorer I get about 90MB/s.

Why is this? The chosen options don't seem to affect the result much.

I noticed three rsync processes one of which is CPU bound at a single core and therefore limiting the performance. This process reads the source files. The other two rsync processes don't consume a noticable amount of CPU time and one of them writes the destination files.

Are there any options to speed up the process? The reason I use rsync is to preserve hard links.

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