Same problem here with %CPU running close to 100% for long periods of time.
I was going to uninstall Avast and move to Sophos Antivirus for Mac when I thought that I would investigate a little bit further and determined that %CPU was the highest when I was navigating files on an external 4.0 TB drive.
To fix this and reduce %CPU from 99.0% to 0.1% I disabled File System Shield, i.e. Preferences > Shields > File System Shield -> Disable.
Interestingly, when I added the volume of my external hard drive HITACHI as an excluded path there was no difference in the %CPU, i.e. Preferences > Shields > File System Shield -> Settings -> + -> /Volumes/HITACHI.
The %CPU was only reduced to 0.1% by completely disabling the File System Shield which of course has a major drawback in not being able to successfully use this shield.
This is now the second time I have
posted this kind of problem using Avast Mac Security so I will probably move to Sophos after I update to Yosemite as it looks like Avast is not as stable running on the Mac as it is on the PC.