Please provide some details of your setup.
Storage: local (RAID level, write-cache enabled, ...), FC (1, 2, 4, 8 GBit/s), iSCSI (1, 10 GBit/s)
Provisioning type: Are the disks you convert thin or thick provisioned?
How is the workstation/serer - on which you run the Converter - attached to the ESX(i) hosts? (virtual machine, physical machine, NIC speed)
Which patch/build do you run on your ESX(i) hosts?
André