Hi
Converter is not capped. Looking for the bottleneck can be tricky. It could even be a disk, not the network, though not so likely.
Have in mind Converter uses NFC to write data. NFC usually uses ESXi's management network. E.g. if management network is uplinked to a slower card...
(On the Hyper-V side, the traffic uses the network the source machine runs on; you have tested it, so it should not be the problem).
But it could be something else, it's hard to tell.
Regards,
Plamen