I installed Avast 4.8 last night after having AVG 7.5 on my Vista computer. ( I did test AVG 8.0 on my XP Pro computer first). The startup is faster than before. Note, however, that I do only load the default and network servers portions (as I did with AVG). All computers, Vista and XP, load either faster or the same time as with AVG.
The only problem I have seen is when the VRDB is built, I allow it to complete the very first time. Then I use the screen-saver mode.