First, I don't agree with the idea (people may have various stuff auto-started with the OS, various IMs, possibly even e-mail clients, and I'd say it's definitely safer to have the virus database updated before these tools connect to the net).
But second, I'd say the OP suggested that the slowdown is because of the updates - and now everyone sticks to that. But I don't see why it should be that way (especially if he says it occurs multiple times a day - I don't believe a pure update check, without any update being actually available, could use much CPU). So isn't it more likely that it's just something else? (scanning auto-started items? if connected to avast at all?)