No issues here after the upgrade, though that's not to say you're not having issues, you clearly are.
First suggestion is to restart your modem/router to make sure its got the correct config file for your connection (it will download it from the virgin network if it needs to). Also, if you're using wireless, try connecting using an ethernet cable and see if things improve.
Failing all that, you may be subject to 'over-utilisation' in your area, and I'd suggest you sign up over at the Virgin community forums (http://community.virginmedia.com/), where Virgin staff can assist you directly.
Virgin Media 30Mb Cable