The Pronto is exactly the same thing as Roundup, only without the brand name price tag. If you'll compare the labels, the active ingredient is glyphosate in both products. The concentration that I get at Tractor Supply is 41%. Click the link below for a .pdf on it from the manufacturer.
www.pbigordon.com/pdfs/ProntoBigNTuf-SL.pdf
How quickly the weeds die are a function of how much product you mix with water. If you need a quick burn down, then mix 3 oz. per gallon to kill within a few days. However, when you do that, the weed will die so fast the glyphosate won't have sufficient time to reach the roots and kill them. Therefore, the weed will come back much quicker. It may seem counterintuitive, but mixing less product, like 1 oz. per gallon, will give you better results. It will take a couple of weeks for the weeds to start dying, but the roots will be dead by the time the weed is and it won't come back for at least a couple of months. And another thing, don't mow the weeds after you've sprayed them. Doing so will cut the sprayed upper part of the weed off the lower unsprayed part, wasting your time and money. If you want it to look nice and mowed, do so before you spray. Also, don't soak the weeds, you're only wasting product there as well. The weeds only need a light mist on them.