CPU heavy alpha shift

So i am building a site for a client, and in it i have incorporated an automatically cross-fading background made of large photos.

Wheneve they cross fade programatically my CPU gets WORKED!

I am using

 if (this.photo1.imageholder1._alpha<100-this.fadeSpeed) {
			this.photo1.imageholder1._alpha += this.fadeSpeed;
		}

as the fading command. Is it just a characteristic of flash that it HATES to do ALPHA changes? Or is there a slicker way to do it?