The most important point in those is the last column of the last two tables - “%util”. It indicates how much of the time the disk (or array) in question is busy servicing a request. For the individual disks in the syncthing only case it’s about 45%. That’s ok, the disks seem to have more to give. But checking the line for md2 it’s at 99% - the RAID array is maxed out. I’m guessing the “lost” performance between that is in waiting. Given that RAID5 needs to both read and write both old data and parity for each write, there will be some time where we’ve done one disk but not yet the the other.
The “IOPS” I’m talking about - I/O operations per second - is what you see in the r/s and w/s columns. The disks are each servicing about 58 + 6 requests, which is consistent with what we expect they should be able to do, being a little bit better than my WD Green guess.
Why so much reading and comparatively little writing? Not sure, maybe this is in the copy phase of syncing?