It isn't as simple as sucking it up for a game or two. I have seen maybe one or two playable games in over 70 online games. I have had as many complete disconnects as I have had good matches. My decent games still have about 1/4 of a second of input lag. 15 frames of being unable to respond is extremely bad. Look at some frame data. If a move had 15 frames of recovery that would be considered terrible. Yet, I am unable to get inputs to respond until about that time. My average game is probably closer to about 1/3 of a second input lag. And my bad games literally run in slow motion (about 50% speed) with the occasional 5-6 second freeze.
There are issues even beyond basic attacking and defending. The movement suffers heavily with that kind of input lag as well. The actual movement feels like you are trying to move through molasses. You can't accurately aim any aerials and you end up overshooting most grabs. You can't move in and out to try to bait your opponent. You basically can't play at any level higher than shield, block, smash, move to the other side of the map, repeat.
My assumption is that Nintendo used TCP instead of UDP. TCP makes sure every bit of data gets to it's destination by sending acknowledgements back. Basically this means both sides have to communicate twice before any input is received. On UDP all data is sent as best effort. Thus, the occasional bit of data may be lost but it works fast. Most games use UDP for their online because it is much faster. That is why in some games, the games run in slow motion and in others they run the same speed but the characters that are lagging skip. They should have used UDP and I believe they did in Mario Kart 8 (I have, on only two occasions, witnessed a character skip in Mario Kart 8). I'd rather the game work that be bogged down by it's own netcode.
EDIT: If my theory is correct and Smash uses TCP, then the LAN adapter should help at least a bit. Wireless signals drop the occasional bits of data. If they have to transmit again and send new acknowledgements, then that would delay the game even further. With a LAN connection it is much less likely that data is going to be lost. Less lost data, fewer retransmits, fewer acknowledgements, fewer delays. At least that is the theory.