I'm almost 100% sure that the fadein and fadeout is lagging because of the size of the image/element.
Check it out here:
Is there another way to make it "smoother" and less laggy besides shrinking the image?
It isn't too bad on my machine (although I do see lag).
When ever it has happened to me, whats usually happening is 2 different 'animations' running at the same time/in same area of the screen.
So it probably has something to do with the fade in and out of the bg glow.
Try removing the glow animation.
I'm not sure this is the actual reason, but from actual experience of this, this is what I'd say it is.