I have my iTunes library on ZEVO CE 1.1.1. This issue has existed at least since Z410 Beta 2011-10-17. It was not reported before because I assumed iTunes was at fault. I realized it must be something else when I found that rebooting cures the issue for several minutes.
If I select an album in iTunes and change a metadata item for all songs in the album, it takes iTunes a very long time (on the order of 2 seconds per track) to write the updated metadata. I've noticed that if I reboot, I can perform the same operations nearly instantaneously (no progress bar even appears), but after a few minutes the I/O starts to crawl again. It is not a progressive deterioration; It's lightning fast for a while and then suddenly becomes slow. I attempted using SSD's for cache and ZIL and it made no difference.
iTunes is also very slow and unresponsive when importing new media. I have not investigated whether this improves after a reboot or when the iTunes library is on HFS+
Unlike another user, I've had no trouble with streaming performance to Apple TV, watching HD movies, etc.