I was involved with a P2V migration last week and while the number of servers to migrate were small the amount of data held on these servers were around 250gb - 500gb in size. This conversion was to a vSphere 5.1 infrastructure using the VMware standalone converter running in a VM. When i started the migration off, we were experiencing terrible transfer rates in the region of 2MB/s and it was saying well over 8 hours of the P2V to complete per server.
After a little bit of googling i found the following article on VMwares communities forum which says that from converter 5.0 and onward, the transfer of data has been encrypted by default and that this could slow down the transfer of data.(http://communities.vmware.com/message/1866091).
I stopped the job and turned off the encryption by editing the converter-worker.xml file and started the job again. The transfer rate, once stabilized went up to around 25MB/s and drastically reduced the amount of time the P2V took!
If you are experiencing slow P2V rate this might be something worth trying.
kind regards and thanks for reading