a few clarifications first:
"SSL set to false." - I assume this means "<useSsl>false</useSsl>" in worker config as in Increasing the cloning performance, 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.