I've been racking my brain on this for the last couple hours, tried uninstalling and reinstalling, manually swapping out java versions, deleting every registry key I could find, to no avail. Then I fired up wireshark and looked at the traffic, and noticed a couple of tcp retransmits coming from the UCCX node. Just on a whim, I changed my guest's interface to a bridged interface, rather than 1:1 NAT, and everything immediately started working as expected (and no more retransmits!).