I'm very satisfied with my DIY NAS. Of course, isn't perfect and isn't for everyone, but i enjoyed the build and i can be "proud" for my creation.
As you can see, SBC based NAS can work very well
But... from my angle of view, RPi isn't good choice for…
About speed: tested with NASTester 1.7. Smallest filesize what this app can test is 1MB.
Tested via SMB share. Results from 4TB WD RED:
(Code, 210 lines)
And 1GB files:
(Code, 29 lines)
CPU & RAM usage while tests: