Well, lag is completely normal, it happens sometimes on the wii u version also. The 3ds version will lag so much more because of the 3ds hardware specs so I would not be surprised if there is no fix to this problem. In any game, lag is pretty normal especially with games like smash bros or call of duty. The 3ds hardware would lag much more because it has a harder time dealing with the online battles, but of course, that is to be expected of a handheld. Like the person suggested a few posts above me, quit the smash fight and join another room, that will help