Don't open captured images in GUI when invoked from command-line

I am repeatedly invoking shutter to perform image captures from the command-line within a script like so:

while true; do
   shutter --window=.*firefox.* -e -o $2-%Y-%m-%d-%T.bmp
   sleep $1
done

The trouble is that shutter gradually runs slower and slower each time its invoked. I suspect that the problem is that each screen capture is being opened in the Shutter GUI as a new tab. How to I prevent shutter from opening screen captures in its GUI? Is there a way to suppress the Shutter GUI altogether and just use it as a command line tool?