Efficient drawing of the background bitmap Garmin Connect IQ

/*
    <bitmap id="bg1" filename="bg1.png" dithering="none">
        <palette disableTransparency="true">
            <color>ffffff</color><color>aaaaaa</color><color>000000</color>
            <color>000055</color><color>0000aa</color><color>0000ff</color>
            <color>ffff00</color><color>ffff55</color><color>ffffaa</color><color>aaaa00</color>
        </palette>
    </bitmap>
*/

var backgroundBitmap;

function onLayout(dc) {
    backgroundBitmap = null;
    backgroundBitmap = new Gfx.BufferedBitmap({
        :width => dc.getWidth(),
        :height => dc.getHeight(),
        :palette => [0xffffff, 0xaaaaaa, 0x555555, 0x000000, 0x000055, 0x0000aa, 0x0000ff, 0xffff00, 0xffff55, 0xffffaa, 0xaaaa00]
    });
}

function draw(dc) {
    if (redraw) {
        // Full redraw, 1x per minute (or when he is looking on his watch)
        var dc2 = backgroundBitmap.getDc();
        bgBitmap = Ui.loadResource(bmp1); // Bitmap 120x120 px
        dc2.drawBitmap(0, 0, bgBitmap);
        bgBitmap = null;
        bgBitmap = Ui.loadResource(bmp2);
        dc2.drawBitmap(120, 0, bgBitmap);
        bgBitmap = null;
        bgBitmap = Ui.loadResource(bmp3);
        dc2.drawBitmap(0, 120, bgBitmap);
        bgBitmap = null;
        bgBitmap = Ui.loadResource(bmp4);
        dc2.drawBitmap(120, 120, bgBitmap);
        bgBitmap = null;
        redraw = false;
    }
    dc.drawBitmap(0, 0, backgroundBitmap);
}