Dryer balls are sold at the larger food stores for about $5 each in the laundry section or on line. They do work (I timed identical loads). Use 6 tennis balls instead and avoid that cost ($5 x 6). Besides speeding up dryer time 30+%, they plump up anything like down coats or comforters I can fit in the dryer loose. I believe they speed drying time because they allow the clothes to separate and tumble more. The down side is the noise and balls falling from the clothes being pulled out. Now I use a pair of pants I was going to throw out. I tie each leg closed at the cuff with strong string, drop a tennis ball in each leg, and tie the leg to capture the tennis ball at the bottom of each leg(remove the zipper so you don’t need to here that noise). This acts like an Argentin Boa that keeps the clothes tumbling. I now only need 2 tennis balls and here less noise. I still use 6 loose tennis balls for down items. One caution: over loading a dryer will not allow clothes to tumble, no matter what you do. Then you end up with those damp spots in your load when you take them out.