Hello, NL staff.
I have this suggestion for adding a post addressing the New Super Mario Bros 2 multiplayer lag. There are several topics regarding this, such as
http://www.nintendolife.com/forums/3ds/new_super_mario_bros_2...
and
http://www.wiiwarewave.com/t2501-new-super-mario-bros-2-lag-d...
The solution for this is as simple as going to internet preferences and turning off spot-pass sending and receiving of data. Of course, it would help if more people can confirm this works. For the time being, my brother and I, and a friend i know can confirm this.
Cheers.