You may find that somewhere in the antivirus scanners options and settings, there is an option (that should be something like) Scan All Files, or Scan Only Executables.
Though, like Calum said, the Antivirus canner shouldent cause major slowdown, you can limit what it scans with its active shield.
A note:
Though this will make your computer faster, it will allow more file types to get in.
Please note that this could allow a virus to get into your system, though non-executable files are not as likely to give you a virus, they still can.