ive always had the fastest growth with anacharis when it was rooted... up to 6 inches a week in the aquarium. when floating, it was more like 1-2 inches a week.
it's all variable to how much light is available and what nutrients are in the water.
I planted some 8-10" anarchis in my tank 2 weeks ago. I use a pea gravel substrate with laterite mixed in. I use flourish once a week currently. No CO2 injection and a 96W 10,000kCF over a 45G tank. Mine has grown about 14" in this 2 week time period. This plant loves to grow. It definitely grows like a weed.