Extremely slow transfer speeds over ethernet

So you are using more then 1 partition.  Is it 2 partitions with 2 drives each?   In a RAID1 setup for each?  1 Data drive the other a clone with each partition?  OR because you're going this 2 partition way, you're going with RAID0, and taking advantage of both HDD for each of the 2 partitions because you are backing up onto the other 2 HDD's, which in effect is a backup.  

Going this route can makes sense, ,but you lose 1 HDD for storage space and using RAID5.  On the other hand, you have a full backup always instead of having to do some type of backup externally.  So my guess is you're going the RAID0 route?

Still doesn't explain the rsync issue.  I looks like you do get some performance hit uisng rsync over just copying for example because of the overhead, but it shouldn't be 4MB/s slow.  I don't have a way to test myself and see what happens.  Looking at this post:

https://community.netgear.com/t5/Using-your-ReadyNAS/slow-rsync-transfer-speeds-NV-gt-NV/td-p/844551

Copying he's getting around 25 MB/s, and using rsync it's dropping down to 1.4MB/s.  That's even worse then you!  I couldn't imagine taking that big of a performance HIT just using rsync.   Is this a issue with a ARM processor?   I would think far more people would be complaining if this was a big issue.  

You need to check your Logs and see if you're having backup errors.  What way is your HDD's formated?  You wan to go into the NAS to System/Logs and click on Download Logs.  You'll get a lot of Data this way.  May want to look at the backup file logs.  Maybe the disc_info log.   Maybe network_settings and look and see if you're getting any RX or TX packet errors.  smart_history looks like a good one.  allso volume looks like a good log.   You should see things like Latest Self Test: Passed on all the HDD's.

All I can do at this point is offer the few tips above.