Javascript slow when running in an UIWebView

Hi there...
I am developing an app for a client of mine, despite the wide array of tools available though objective c, they (the client) has insisted that we use rudimentary web technologies (HTML, Javascript etc) for the program, I believe they are looking to save development costs on ports to other platforms.

I wrote an animation routine in javascript which runs great in Safari on the desktop but very slow in both iphone and ipad (code included below) we were able to resolve a graphic rotation (rotating a large picture clockwise) issue by using css3 animations and key frames in the webkit, but this animation which consists of a series of bitmapped images (png files) was not supported that I could find. Any advice would be greatly appreciated.

Thanks

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var dieNum;

function dieLaunch(){

// var dieImageSet5 = 19; // total 83

dieNum = 0;
dieTimer = setInterval("dieAnimation(5,19)",40);

}

function dieAnimation(set,end){

var fNameNum;
var fPath;

if(dieNum<=9){<br /> fNameNum = "0000" + dieNum;
}else{
fNameNum = "000" + dieNum;
}

fPath = "url(graphics/dieImages/" + set + "/roll_" + set + "_" + fNameNum + ".png)";

document.getElementById('dieStage').style.backgroundImage = fPath;
dieNum++;
if(dieNum > end){
clearInterval(dieTimer);
movePlayer(dieRand);
}
}