avast.setup is the auto update process downloading VPS updates, it is set to a lower than normal priority, so as not to be a system resources hog, if it is only taking 30% of CPU, what is using the rest. Unlike other AV updates, the avast VPS updates are incremental so should be smaller and take less time.
I do notice a little slowing when the update process is running, but the problem may be that it happens very close to your boot activity (once an Internet connection is detected), so that time could be quite busy for the system and to have 30% taken up would be noticeable.
I take it that this happens shortly after boot ?
If so I would suggest you delay the avast auto update check.
You could edit the avast4.ini file, [InetWD] section, add the following line if it isn't present (or edit if present), AlwaysConnectedWaitSeconds=120 and AssumeAlwaysConnected=1 (for broadband connections) or RASWaitSeconds=120 and UseRAS=1 (for dial-up connections).
See example below, both these values wouldn't be present at the same time obviously. The figure is in seconds and you can set your own duration to wait before checking for updates. This allows time for a connection to be fully established.
[InetWD]
AssumeAlwaysConnected=1
AlwaysConnectedWaitSeconds=120
OR
UseRAS=1
RASWaitSeconds=120