Well that is intentional, if it is set to a higher priority it could well impact further on your system performance, particularly on the first check after boot. Once a connection is available, avast will check for updates and that can happen relatively early on in a boot, which could slow the overall boot duration.
I don't believe that there is any way to increase the priority in the UI or in the Task Manager as the self-defence module would most likely have something to say about that.