It seems that running a NFS client in one of the DomU (guest OSs) results in VERY poor read/write performance.
So I’ve had to run everything over a Samba connection.
Not the prettiest solution, but I get normal read/write speeds.

This is weird, cuz this worked just fine in Suse10.3… Or did it… I never tested it, but I also never had any reason to suspect it as not working, since speeds were very decent.