Hello SL1000,
in the meanwhile I guess I have solved the problem(s).
First, it began to get worse and even the smallest files could no longer be opened; so I did a hdd check and it appears that one of the brand-new hdds is defect... argh! So I replaced it. However, the performance problem remained, and data transfer rate remained extremly low (about 5kB/s). Then, I decided to turn everything off - not only the NAS, but also my WLAN router, and voilá! After rebooting the WLAN router the data transfer rate was again as I expected. Somehow, the NAS and my router are having communication problems. I got a new router on Friday and since then everything is working fine.
Now I'll begin to restart the services and protocols I need and continue to observe performance. Hopefully, I won't bump into new problems any time soon...
Thanks again for your help!
brazza