Synology DS411slim: getting slower with time?

I’ve written in the past a few posts about this small nice NAS: here, herehere and here.

Yesterday I did another test after updating the complete local copy of my gmail messages on Thunderbird.
It took 4 hours 33 minutes to compress 11,548,610,909 bytes (578 Files, 61 Folders) to a single 7zip file of 7,108,652,903 bytes: it is a lot of time.
16.380 seconds of runtime means an average read/write speed of 1.139.027 bytes per second.

Both the NAS and the computer I used are connected with 1Gbit copper ethernet to the same switch: during the process the usage stats on the windows 7 (64 bit) machine indicated almost no load and the same was true for the NAS according to the web interface.

The DS411slim is running version 4.2-3202 with 3 HDDs in a single disk group using SHR (Synology Hibrid RAID)

This is really puzzling. And a bit upsetting.
Any suggestion to recover from this bad performance is welcome.