I'm using the latest version of avast at current time of writing on windows 7 OS. I found out when my internet connection has problem (usually hinted by exclamation icon on the wifi icon), opening image files like jpg file with image viewer application is very slow (could take around 13 seconds for 165 kb jpg file). I thought the image viewer app was trying to connect to internet everytime it runs, but I can't found settings related to it, and it doesn't make sense too for the app to connect to internet for every load, otherwise it's terrible app.
So then I thought could it be avast ? Then I played with some avast settings and found out the culprit, when I unchecked the "Enable reputation services", the problem disappeared. I guess by enabling the reputation services, avast will try to connect to internet when scanning the opened file/app. I would like to suggest to improve the reputation services feature on bad connection situation, maybe if avast detect connection issue, it should disable the online database querying when scanning.