CSV allows you to have the disk (volume) owned by one host (physical server) while a guest (VM) on there is owned by a different host. To allow this to work uses the CSV network link. If this link is slow, your disk access is slow. Moving the guest and CSV onto the same host means it doesn't have to go through the CSV link to be able to write to the disk, and therefor it may be faster.
This is why I asked if you had verified if which host owns the CSV and if that's the same host that owns the guest that you're testing performance on.
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