What can cause 90 second delays joining a wifi network?
You can also boot up to the start up disc that come with your mac if it is 10.6 of newer. There you can connect your wifi when the disc has loaded up to check make sure its not a software issue.

You can also try resetting the P-Ram on startup, by holding P + R + cmd + alt and letting its chime 3 or 4 times. That may help.

Also check with other machines to see how long it takes to receive an ip address. This may be an issue with the router.