My email and browsing don't appear to show any appreciable slowness, the strange thing, me on dial-up.
If this also extends to avast's auto updates I find that as a) it is incremental and measured in KBs rather than MBs, b) the auto update also use a tweak to restrict the CPU% used in the update, so there is most certainly something wrong.
However I don't feel a reinstall is the way to go as there may be something else in the mix that won't change with a reinstall.
What is your OS ?
What is your CPU and how much RAM do you have ?
What is your firewall ?
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
Leave that VRDB at the default setting, it will run every three weeks (21 days), but for the first time click the Generate Now option. Do that when you aren't going to be busy the first time might take a while, depending on your system spec, and how many executable files you have on your system. It doesn't copy them, but takes info on the file to assist in repairing them if infected.