New to avast today. I have installed on a newly rebuilt computer running windows 7 and 2.3ghz processor. Great speed and very happy. I also installed on a brand new computer running windows 8 but only a 1.7 ghz processor. I am very surprised at the speed difference of a complete scan. The brand new computer is very slow scanning. I have uninstalled the norton software, but very disatisfied with the speed. Would the different processors really make such a huge difference. Complete Scan takes almost 4 times longer. Thanks for the help.
Processor speed, ram, the amount of data being scanned will all add to the amount of time that the scans require to complete, is this the initial first scan on the windows 8 system? the first scan always takes longer as the cache needs to be built so subsequent scans should get quicker.
Welcome to the forum. 
Did you fully remove Norton ???
The following may be of help to you:
http://goo.gl/VLXde