performance html 5 canvas raster

HTML

<canvas id="c" style="z-index:1;"></canvas>

CSS

canvas {
    overflow:hidden;
    position:absolute;
    top:0;
    left:0;
    width:100%;
    height:100%;
}
html {
    width:100%;
    height:100%;
    overflow:hidden;
    position:absolute;
    top:0;
    left:0;
}

JavaScript

var can = document.getElementById("c");
var ctx = can.getContext("2d");

can.width = window.innerWidth;
can.height = window.innerHeight;

w = window.innerWidth;
h = window.innerHeight;

var img = new Image();
img.onload = function () {};
img.src = "http://placekitten.com/500/500";

function redraw(mouse) {
    ctx.save();
    ctx.beginPath();
    ctx.arc(mouse.x, mouse.y, 100, 0, Math.PI * 2, true);
    ctx.clip();
    ctx.drawImage(img, 0, 0, w, h);
    ctx.restore();
}

can.addEventListener("mousemove", function (e) {
    var mouse = {
        x: e.pageX,
        y: e.pageY
    };
    redraw(mouse);
}, false);