Image Processing Makes Livecode Appear to Crash

[Ray]

Fellow LC Users,

I'm trying to provide users with a 'magic wand' style tool.  Click an 
image, pick up the color clicked and turn every pixel of that same color 
in the image transparent.

The loop I've written takes about 30 seconds to run on a 600 by 700 
pixel image, which is a problem, but worse, click anywhere before it's 
finished and Livecode 'appears' to crash.  I say 'appears' because the 
cursor turns into the crash cursor and the title bar displays 'not 
responding'.  After the loop is done Livecode returns to normal.

I'll include the loop, below.

How do I stop LC from appearing to crash while running this loop, or how 
to speed it up.

Thanks,

Ray
LinkIt! Software

on mouseUp
    lock messages
    put the mousecolor into myColor
    put the mouseloc into myLoc
    put the imagedata of img 1 into totColorData
    put the alphadata of img 1 into totAlphaData
    put numToChar(item 1 of myColor) into binColor
    put numToChar(item 2 of myColor) after binColor
    put numToChar(item 3 of myColor) after binColor
    put numtochar(0) into bin0               -- transparency
    repeat for each char curBin in totAlphaData
       if char 2 to 4 of totColorData = binColor then
          put bin0 after newAlphaData
       else put curBin after newAlphaData
       delete char 1 to 4 of totColorData
       set the cursor to busy
    end repeat
    set the alphadata of img 1 to newAlphaData
end mouseUp