Thank you Hans for searching how to accelerate a Java program, but pointing to an article which is 13 years old (an eternity in computer programming) couldn't lead anywhere. Java has made leaps in performance since that time. [:)]

Anne, you should check first that when a photo is rendered at one of the two best quality levels, there appears 6 colored squares during the rendering process since your processor has six cores. Then, you should try to increase the max memory used by Sweet Home 3D to 1 or 2 GB; this should really help. Finally, you could try the Java 7 as soon as a beta version will be available (this should happen in February).