After the successful install I am also getting speed between 7-8mbps which is very low for my needs. I do not have the time and patience to go through troubleshooting so I have to sell both WD hard drives.I recently tried TrueNas on a old laptop (i5 5th gen, 8GB RAM, 240GB SSD for OS, External hard drive on USB 3.0 for data) but I wasn't impressed with the results since the data transfer speeds I got was around 4MBps. I used this link to help set-up the system. The guide is super simple to use.
Make sure you try out TrueNas first on a spare harddrive first, see if you are getting decent speeds on file transfer..if that works out, then you should be good to go! I didn't troubleshoot the reasons for slow speeds further, you might have better luck!
I have the feeling that NAS will be too complicated for my system and me and I should stay away from it.