I keep my data on a single JFS drive, including my PMMail test mailstore, which has over 1 million messages. I back up this drive (2 TB) to several other drives on other computers using Rsync in client/server mode.
Due to a drive failure on my main computer, I had to use Rsync to transfer the entire drive, which took 30 hours. This seemed long to me, so I experimented with things and found that the PMMail mailstore was taking most of the time. Not only that, but the server was so loaded down that I could not use it.
Further experimentation showed me that it was the Extended Attribute (EA) processing that was taking so long. I used eaclean, by David Azarewicz to remove the EA from my mailstore message files and the time to transfer the mailstore using rsync changed from 9 hours to 45 minutes. The server was no longer impacted and the user interface ran normally.
You can obtain eaclean from David's Software Download Center
https://88watts.net/software.htmleaclean was able to remove unnecessary EA over one million files in a few minutes.