I'm unable to solve this and hoping you might be able to help me. One day, seemingly out of nowhere, my transfer speeds went from ~30MB/s down to ~3MB/s. After lots of testing, I found that the RAID5 attached storage is the only thing that's slow. iperf is fast, writing to the sdcard is fast, but writing to the external hdd is painfully slow.
I've taken the RAID out of the system and connected it to another computer. In so doing, I was able to get speeds of +120MB/s so it's not the array itself.
I've also tried a new image to see if any of my settings somehow borked the esata connection but experience the same abysmal speeds.