Hate to necro a two-year-old thread, but... so far, I'm mostly ok when I play. Match me up with someone in the UK or Europe however, and I am completely screwed (literally a second of input lag!) And that's 1v1... put me in with 4-player FG of any kind, and it lags a bit worse, although it depends on who I end up with (usually the mighty Japanese... for some reason, Australian SSB4 carts prefer Japan over other regions, even it's own!

)
And local seems to depend on which 3DS systems are connected, I've had a couple of odd hiccups when playing with people who have the new 3DS/new 3DS XL, but otherwise, local's pretty good.