I'm using converter standalone 5.5 to transfer VMs between two vSphere clusters. The source is 4.1 and the destination is 5.5. The reported transfer rate is about 10 MB / sec. The hosts, storage and network end to end are capable of much higher transfer rates. If I do a standard windows file copy from a guest on cluster A to a guest on cluster B I'm seeing about 100 MB / sec which is what i'd expect since the LAN connection is 1 Gbps, so the bottleneck seems like it has to be somehow related to VMware.
I've noticed that if I browse to a datastore on a host and upload a large file (e.g. an .ISO), that transfer rate is also much slower than what I would expect. Is VMware doing some sort of throttling? Is there something I can do to improve the transfer performance?