For a tiled repeating background, I would recommend looking into createPattern function https://developer.mozilla.org/en/Canvas_tutorial/Applying_st....

You can use code like this:

    context.fillStyle = context.createPattern(img, 'repeat');
    context.fillRect(0, 0, 100, 100);

Where 'img' is an Image object you loaded from an src, or even a canvas object that you may have used to build a composite background.

If this doesn't work in your situation, I would at least build this image once in a canvas in memory (using document.createElement), and reference it in the 'drawImage' function instead of building it from scratch each time. Though it sounds like you might already be doing this. Do you have a link to your game?