Hello
a few clarifications first:
"SSL set to false." - I assume this means "<useSsl>false</useSsl>" in worker config as in , correct?
"Ensured NIC speed set" don't know what you mean by that, hope it's ok.
Other factors that affect performance might be
cloning level - file level is slower than block level; file level will be triggered by shrinking the volume or changing the cluster size
ESX host performance. The conversion establishes a connection between the source machine and the destination host and uses the proprietary NFC protocol. I.e. the host current load, disk speed etc. will affect the performance, too.
HTH
Plamen