My TiVo box is really really slow too, there are a few thing to try that may help make you box run a little faster, it is only temporary though and the box slow again in a daybor two. I too am considering leaving Virgin Media because of the speed of the TiVo box, this has only been an issue since the last system update. Virgin have also just increased their prices which means we call all leave mid contract.
I've done all the obvious re-booting and refreshing but you are right, within a day or two it goes back to working like a steam boat rather than a fighter jet ... or should the be The Bolt!
At any rate, a few friends have taken the advantage of being able to leave and have invested in YouView boxes and are happier now. I just don't want the hassle when the service should be right in the first place.
Im currently in discussion with my landlord about getting the dreaded dish. I really hope Virgin sort it out as I really like what they do. But this TiVo issue is silly, I can make a brew in the time my box takes to respond.
An engineer said it is because it gets hot being on all day.
As for a V6 box - you need to register an interest to try and get one. I have done so already but I rather suspect there is little chance of it coming to my house, despite me having a 4K TV to take advantage of its features.