Since a couple of week on VMTN forums a new thread is collecting much attention from most experienced VMware users.
A customer posted some benchmarks after testing different SAN I/O operations with ESX Server 3.0, revealing a serious performance degradation appears when a virtual machine is moved from a LUN to another.
Other notable experts confirmed same behaviour and also notified general performance of VMFS3, the new file system introduced with this release, went worse during the all beta cycle.
Even if the issue seems to appear in more than one scenario VMware, monitoring threads on forums, recognized the problem depending on VirtualCenter and it’s already working on a fix.
If you have experiencing same problems help to deliver a better patch opening a support request and posting your findings on the original thread.