I have run across a couple devices that just don't seem to scan well, don't recall if defy was one.
Maybe try (barcode scanner zxing based one )from the play store and see if that has slow scanning issue... If that can identify real qr codes looks like your developer mucked something up.. If it does not work, then the defy has an issue... Try another phone...
Without even seeing the code -- hard to say anything. QR codes scan virtually instantly. It's likely there's some misunderstand here -- like you are not actually using a QR code.