There is an alternative way to write directly to STDOUT:

print "Content-type: image/tiff\n\n";
binmode STDOUT;
$img->Write('tif:-');
[download]

I thought this might have saved some time but when I tested it on a 75Mb tif image it took 3 seconds using both this method and ImageToBlob(). I guess that's just how long it takes to write the image.