This is actually quite a common complaint, I know I have had my struggle with the TiVo. A reboot usually helps for a while. I found that it usually gets worse if you use any of the apps (Netflix, Vevo, Etc...). Thats the only real Fix I have found. Usually wait until it is so slow pressing home or guide blanks out the screen for about 20 seconds before I reboot. Can do a reboot from the Home Menu > Help & Settings > Clear or reset and Select Restart. The others will wipe out settings and recordings, and only do it when something isn't recording or about to for about 15-20 mins as it can take a while to boot.
Hope a reboot helps... and also hope that Virgin do something about the constant slowness of the box!