Converter 5.0 is slow, and doesn't work with ESXi 5.1 yet, so don't try it! Converter 5.1 is due for release in November I believe. For slow transfers with ESXi 5.0.1 or below, see the following text.
SSL is the issue, VMware have switched it on by default - for a much higher transfer rate, disable SSL.
Here is how it is done:
Open the converter-worker.xml configuration file. It is located in "%ALLUSERSPROFILE%\VMware\VMware vCenter Converter Standalone" folder for Windows Vista or newer or in "%ALLUSERSPROFILE%\Application Data\VMware\VMware vCenter Converter Standalone" for older Windows versions.
Set the key Config/nfc/useSsl to false and save the configuration file. Restart the "VMware vCenter Converter Standalone Worker" service.
It should look like this:
... <nfc> <readTimeoutMs>120000</readTimeoutMs> <useSsl>false</useSsl> ...