AB testing shouldn't effect your load times.
Make sure your images are saved the exact size you are presenting them on your site at 72ppi.
In other words, if your image is Actually 800 x 600 pixels and it's saved at 150 pixels per inch, but you are displaying it at 400 x 300 on your web page, it will load slower than necessary.
Your computer screen can really only show you 72 pixels per inch resolution. If a picture is only going to be displayed at 400 x 300 pixels, save it that size.
Does Google Website Optimizer slow down webpage loading times?
