For the record, I have never worn a sundress and the only time I breastfed, I was the recipient.
That being said,surely they make quick dry ladies swimwear? I buy the quick drying board shorts and am 95% dry by the time I finish rinsing my gear.
I suspect the issue is that the swimwear is stuck to our skin so there's no opportunity for air to pass through the fabric. Even the thinnest fabric will take longer to dry if it's right up against your skin verses loose. And if our swimsuits were made of that quick-dry fabric I think we'd be displaying more than some of us would like (and more than some would like to see, in many cases).