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>
...