I have had the wii lag horribly when i first got the game but after opening ports by either port forwarding or putting the wii's ip on DMZ every match has been smooth.
Follow instructions below.
The wii's ports are 28910, 29900, 29901, 29920, 80, and 443. If you know about how to port fowarding then use those ports.REMEMBER YOU NEED TO KNOW YOUR WII'S IP.
Either google "how to port foward" or "how to DMZ". For me DMZ is the easiest.
This is what i did for DMZ
I turned the game on and continued like i was connecting to a wifi battle like normal. After the part when it says connecting.
I went to my pc and logged into my router (normally http://192.168.1.1). When i logged into my router(Linksys) I went to Wireless then wireless MAC filter then edit MAC filter list then wireless client mac list and it was there under active PC. It should say 192.168.1.***
Or you can type in http://192.168.1.1/wclient.htm to see it
I then went to the tab Application and Gaming then DMZ and put the last set of digits after the last point and enabled it. I saved the settings and now all ports are open on the wii. That is what DMZ is for. It opens all ports for a specific IP. It has been night and day. Everything is smooth.