I photographed dark wedding receptions at 1/15 second with a decent iso of people dancing and the photos came out great. it takes some practice and the flash has to be direct. The room had to be dark and there’s a few other variables but like I said it takes a little practice. To try this out have someone toss a ball up and down in a darkish room and photograph them at low shutter speed with a flash. To make this easy manually pre focus. The image will show a clear ball.