I had the following from Virgin Media when I asked about whether the TiVo's speed is something to worry about:
TiVo is very much in beta stage at the moment and is continually being worked on. The TV team are working hard to iron out any glitches and problems with response time. Going forward you should expect to see improvements towards a truly smooth and slick service.
Reassuring I'm sure you'll agree, here's to more functionality and more speed!