You need to edit (using notepad) the [InetWD] section of the C:\Program Files\Alwil Software\Avast4\Data\avast4.ini file and add the following line (depending on your connection type enter one or other of the Bold lines below):
Dial-up connections, add this line: RASWaitSeconds=120
Broadband connections, add this line: AlwaysConnectedWaitSeconds=120
When complete save the changes, avast's self-defence module will ask for confirmation, etc. answer Yes.
The figure is seconds and the above equates to two minutes, you could try that and adjust upwards if required, 180, 240, etc.
I'm on dial-up and I set mine for 10minutes, 600 seconds this allows me to connect to the internet collect my email or quickly do something and go off line before the update check takes place. If I'm on-line for longer after 10 minutes avast checks for updates, but it only downloads VPS updates if you have it on Auto (you can change that, but I wouldn't recommend that).
The auto update restricts the amount of CPU% that avast.setup uses, so it shouldn't really have the impact you mention. The update process has also been improved recently, so I would suggest that you ensure that you have the latest version 4.8.1290.