If this is within a single ESXi host you should have a look at using SSH or the unsupported console to copy between datastores. You can also try using the datastore browser. There is a "Move" selection. Select the root folder of the datastore in the left hand pane and right click on the VM folder in the right hand pane. Select Move and browse to the destination datastore.
Your speed will be dependent on the quality/speed of the underlying storage controller and drives.
