Yes, Cynthia mentions that too. Plus the sun whitens. I do my older white fabric items this way.
I tried soaking in bathtub, but didn't care for the process. The dirty water is difficult to drain well and remove as much as possible from the fabric you're cleaning.
I get one of my big plastic bins to place in tub. Makes rinsing with fresh, clean water much easier. Squeeze like a milkmaid then lay out on grass.