I also want to add my name to the list.
TS-410, 4x 2TB HDD, RAID 5. Problem: Upload extreme slow, Samba and FTP timeout. Download/Downstream ok.
I also recognized flush 9:0 acting up (so I found this thread), but the more intense problem was that after trying to upload a file the smbd process remained and ate CPU and RAM. 2nd try to upload: 2 smbd processes, 3rd 3 and so on. Just a NAS reboot could kill them.
I tried your solution and it worked: when free disc space falls below 425 GB (7,7 %), upload is not possible anymore. If you delete some files, everthing is fine again. The problem is reproducible: the magical space limit of my NAS is about 425 GB free disc space.
The problem occured suddenly, so it could be a combination of "low" disc space AND firmware update, but that hasn't to be true.
I had a long discussion via e-mail with german QNAP-service, they really don't know something about this problem.
Another thing that attracts attention is that most (or all?) the people here having this problem are using a RAID array, most of them RAID 5. Some are using the same QNAP-type TS-410.