optimize shell script (snapshots)

I've a script to do some snapshots but the time it does so is very different...
once i got a snapshot under 1 sec, on the other hand it took 3 sec, but nothing else changed, i didnt even move the cursor or something.

I put the script on a ramdisk and its faster, but still swing from under 1 sec to 2 secs. What else can I do to make it faster or just steady?

until [ $A -eq 1 ] 
do 


        ## TIME
        START=$(date +%s%N)
        ##/TIME

    xwd -name "Windows XP Professional"  | xwdtopnm 2>/dev/null | pnmtopng > picci`echo $COUNTI`.png

    echo "picnumber "$COUNTI




    COUNTI=`expr $COUNTI + 1`
    if [ $COUNTI -gt 250 ]
    then
        COUNTI=0
    fi    

            ##TIME    
            END=$(date +%s%N)
            DIFF=$(( $END - $START ))
            echo "It took $DIFF seconds"
            ##/TIME
done 

It most likely has to do with resource competition - another process is using the disk, or has the cpu, whatever. Other times, you are first in line for all resources.

The only thing I know is to bump the priority of the process. Since you do not set A=1 in your code, it will loop forever. If it does that at a very high priority you have a BIG problem.