Ok, I ran a packet tracer on a switch with a mirrored port. Here's what happens
You try to start up a game, you contact a nintendo server for the IP of the person you're trying to play against. On a few ports ranging from 50000-65535, connection attempts are made. If they fail, nintendo's NAT connection helper server is contacted, which initiates a UDP connection between it and the wiis of the people who are trying to play. The IP is then changed to the IP of the remote Wiis, and by crazy UDP hole punching magic, you're now directly able to communicate to the other wii WITHOUT THE USE OF NINTENDO SERVERS. BEYOND THE INITIAL CONNECTION, NO NINTENDO SERVERS ARE USED
This is a problem with the code, and our infrastructure. I have a ping of 18 between myself and a friend, but the game is lagging behind on inputs.
There are things you can do to fix this.
a) Try lowering your MTU, I'm currently running 1346 and it's working well. The game slows down a bit from time to time, but the controller input is accurate within milliseconds, it feels like I'm playing local. Packets can get combined and received as one with a larger transmission unit, depending on OS, switch type, router hardware and os, etc. If a packet is sent with a larger size than some hardware down the line supports, problems and fragmentation can occur.
b) Try a wii wired adapter, or at least try disabling things like frameburst on your router. The wii is not compatible with all routers, because a lot of routers choose not to conform strictly to standards.
c) Try port forwarding ports 50000-65535. For me, it seems the ports that people use to send UDP packets to me on falls within this range, and they're usually the same ports. I'm not sure why this is, but I don't think the ports are entirely randomly generated. This _probably_ won't speed things up, but it will allow you to connect faster, and stop you from receiving errors since you won't need to use the nat helper server. Your mileage may vary, if you want to do your own investigations, put your nic into promiscuous mode, put a HUB or switch with a mirrored port between your PC and the wii, plug it into the router, and run the sniffer and see what magic happens.
Basically, you and a friend can try going here
http://whatsmyip.org/
Then start, run, cmd, in the box that pops up, type "ping (friends ip)", without the ()
If your ping is < 100, in my case, I get 18 with some local friends, there's a good chance of getting gameplay working more responsively. If not, and neither of you are running torrents, maxing your connection, and triggering your ISPs CBQ/QoS/limiting causing increased ping times...there is nothing that can be done if that is the case.