The update schedule is like this, when you first connect to the internet avast will check for updates and if you remain on-line it will check after four hours and again fours after that. So there is no fixed schedule so it is a co-incidence if it falls within your work day (around 4:30 PM Eastern).
Which brings us on to the fact if you are using your computer for work you should be using the Pro version and not the free Home version which is for Home and Non-Commercial use and both must apply. The Pro version has push updates which may be more suited to how you work.
However, on XP, Vista, NT bases OSes the auto updates are CPU restricted so that they 'don't' slow your computer and the updates that are likely to happen around once a dat are the VPS (virus signature) Updates and these are measured in KBs rather than MBs as the updates are incremental.
So I'm not really sure what is going on on your system as I see no slowdown on the VPS Update when set to auto.