@Rob Not sure if there's any performance tweak I can suggest, it is very intensive dong that conversion. The VoltaicHD website suggests that it take about 12 times the actual length of the clip to convert, so what you are experiencing does sound like an overly long time to convert. Check the Voltaic FAQ for more info:

http://shedworx.com/voltaichd-faq-pc#1

Drop them a line and I'm sure they'll be keen to figure out what's going on!