Hi kautilya, if you think virus scanners are fast then just take a test of the open-source AV called "clamwin". It's really slow, I think it really does scan every file in its entirety.

Why I think that most AV's are fast is that they are probably smart enough to exclude many files from requiring a complete scan. I would guess that with certain file types (say a wav file for example) it would be sufficient to just verify that the file-header matches the file type correctly and then just declare it safe without further scanning.