I still have logs from some of migrations I performed few months ago, it was quite a bit faster than that, at ~30 to 40 MB/s, as measured by rsync --info=progress2.
And the final sync was a matter of minutes.
Maybe your storage has additional constraints that make operations slower?
As long as your extent size is of decent size (the default of 4 MB is ok), it’s pretty much raw sequential speeds.
Would probably be suitable enough if this suggestion was implemented.