You may want to try resetting CBT on those machines. If you're doing incrementals on a 900 GB file server which experiences a change rate of 0.3-0.5%, it should be taking nowhere near that amount of time, even given poor performance, unless it's rescanning the entire disk. Also, I hate to break it to you, but VDP never was a good performer to begin with so there may not be a whole lot from an appliance perspective you can do. You do get what you pay for...
Great, thank you!
After CBT reset, now backup of our fileserver takes from about 1 to 4 hours max, but the 1st backup (full backup) took 39 hours!
We've set a new VDP appliance for test, and we have separated another DS1513+ NAS, to check the "performance analysis" in the fresh VDP... but it's failing everytime (time exceeded).