Update, Just ran a Full System Scan whilst working on the forums.
Time 0:10:56
Files 98987 Folders 7461
Data tested 26.6GB
Now this may actually have taken a bit longer as it alerted on some spam/phishing database files. Once avast makes a detection it automatically switches to high sensitivity, which could mean that the scan takes longer. However, I have all of my scans set to High Heuristic Sensitivity anyway so perhaps it didn't have so much of an impact.
Edit: I don't know how far different my core2due CUP is from yours, but it doesn't seem to effect my system as much as yours. As avast says this could have an impact if you have lots 'large' files, which I probably don't have lots.
I don't know what your scan settings are, but here is a suggestion:
In the scan settings > Performance > Persistent Cache, enable the 'Store data about scanned files in the persistent cache' (I think this is off by default) and the 'Speed up scanning by using the persistent cache' (I think this is on by default). This after a few scans should populate this cache and speed up scans.
Scroll down a little in that same screen and on the File access sub-section try enabling that option, this should improve scan speed.