Today I discovered that one of my domUs at work is performing slow on its mounted NFS share. Bonnie++ and dd tests showed a network throughput of just 300 kB/s whereas the throughput was up to 110 MB/s from dom0 to NFS server. Some Google searches revealed that Xen has problems with NFS performance with non-standard rsize and wsize setting and especially with NFS over UDP.
Reducing rsize and wsize settings didn't help at all. The performance was still awful. After remounting the NFS share via TCP the performance was as expected. Such a huge difference in perfomance surprised me. Still, a strange bug in Xen, at least in Etch. Maybe it's already fixed in Sid?