Do a couple of reboots, whit a new installation of avast5 during the boot it creates/populates the persistent cache.
The purpose of this persistent cache is to speed future scanning, but the overhead is a slightly longer boot/initial scans. However, what you are experiencing is certainly not the norm, yes I can imagine it might take a minute or so longer (depending on your system spec CPU, RAM, etc. ?) but nothing like this.
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
What other security based software do you have installed ?