Avast/AVG/Comodo scan speed: too fast?

I'm running AVG Free on one computer, and Avast on another. Today I also tried Comodo AntiVirus. All three are configured to do on-demand scans only. No background scans, no on-access scans, no scheduled scans.

An on-demand scan of a 1.1 GB movie (avi) takes ages with Avast.

Comodo takes ages (well, seconds) to load its scan engine, but the file itself is scanned in about a second. AVG takes less than a second to tell me the file is clean.

Sounds good, but...

...the scan is too fast!

It's simply impossible to scan all 1.184.337.920 bytes of my movie that fast. I know for sure that the read speed of my HD is nowhere near a gig per second.

So I suspect that AVG and Comodo take shortcuts. Maybe they only scan part of the file, like the headers, but not the video frames.

Can anyone confirm this?

 

avi's, if they are actually avi by content might be skipped by some scanners because they are not programs. most AVs scan it quick though.

 

avast!'s right click context option scans the files in most thorough way possible.
This means it will unpack any archive, any packer, scan entire file from start to end regardless of file typeand will also ignore virus targeting detection.

 

Is there a way to make AVG do the same? I looked in all the config sceens, but I couldn't find any option to make it scan the entire file.