try using smartctl -a /dev/sda (or /dev/“harddisk location”) and see what the health status is. If you are using GUI on linux, there’s probably some smartmontools (eg gnome-disks) that shows you the same thing.
A dying harddisk could have symptoms of taking forever to read but I hope your HDD is still healthy.
Hope this helps.
Frankly I didn’t help much but I would also like to know if you have managed to find out what the issue was that’s causing the slow sync? Didn’t expect your HDD to be SSD instead which shouldn’t be causing the slowness?
I didn’t manage to find a solution to the slow sync with Rclone other than switching to Rsync for the part to sync from my internal partition (SSD) to my external drive (HDD). From there, Rclone takes it to Backblaze B2. I don’t know how fast or slow it syncs to B2 because I have almost 1TB and I expect it to be slow…