Hello everyone!
After a long time I was able to work on the NAS, and I managed to solve the problem!
The problem was a faulty BIOS, made for a slightly different version of my NAS. I installed it after reading on the internet that it should've fixed some problems when using OSes different than the one it was sold with, however the CPU management was completely broken. After finding and installing the original BIOS now performance are what I would expect.
Indeed I reach around 100 MB/s both when reading and writing using NFS over a 1gbps link.
