This is very true, but I don't know if you want to add duck weed to your tank. It has taken me forever to get rid of it. Frogbit is a much nicer floater and grows quickly.
As to the op's question, it depends on your tank. Dwarf hairgrass spreads like the plauge once it is established under good conditions. So does Dwarf Sagattaria. Ludwigia grows fast as well and is a very good easy to grow plant. It is a water column feeder but you can take cuttings and plant them around the tank if you want.
I a couple good choices are dwarf baby tears and java moss.Java moss is a lot better with co2 but baby tears don't need it.Also microsword is a good choise these are all fore ground plants good fr carpet plants which are very beautiful.
^ i think you must be thinking of another plant other than dwarf baby tears, since that is one carpeting plant that is notoriously difficult to grow unless given ample pressurized Co2, high light and good fert dosing...
this question really depends on a bunch of factors. all plants will multiple moderately rapidly but it depends on the conditions you're keeping them in