OK so I had both my 3DSes on the same access point.
I started off with having both Mario Kart 7s join in a racing community. The result was that there was no lag, slow downs whatsoever during the beginning and throughout the races.
So next I proceeded with having them both join a battle community and again no lag or slow downs occurred.
So finally I proceeded with having them both join online free-for-all races with random people (some with VRs up to 10000 & up.
) The beginning and through the races I experienced no lag, slow downs, nothing. The frame rate remained unchange and smooth as always on my end.
I have Time Warner Cable internet (Road Runner internet speed I think
) thats hooked up to my Linksys E2500 | Advanced Dual-Band N Router. (Wifi Router :p )
Are you using one of the more newer routers or is yours a more older one?