A decent metal cutting blade should go through a leaf spring.
Are you applying maybe too much force when cutting ?
Make sure you allow the saw to cut it's way through, you won't speed it up any by trying to push it through...and if that's the case, could be just that you overheated the blade.
Try a new blade, go easy on it and see if it works.
Some type of cut-off wheel would be the quickest, either chop saw or angle grinder if you are just making a couple cuts across the width, you'll be done before you even get your forge lit to hot cut it...But if you're trying to split it down the center length-wise or something like that, you'll likely want to throw it in the fire and hot cut.