My wi-fi suddenly started crawling last night, even the smallest web pages took ages to download and spotify kept stopping and starting. Going to broadbandspeedchecker.co.uk showed that I was getting a mighty 0.03 Mb/sec! Before I started railing at my ISP I connected my laptop via ethernet and suddenly, BOOM! 15Mb/sec. So it seemed pretty obvious that my wi-fi was getting shafted somehow and the prime candidate was channel contention.
After a quick dig around for an app, I stumbled across something even better. OS X has a command line utility that allows you to see what channels are in use by the wifi in your area – the airport command. The following (it needs to be all on one line) will let you see what wi-fi endpoints are being broadcast and what channels they are using.
/System/Library/PrivateFrameworks/Apple80211.framework/ Versions/Current/Resources/airport -s
It immediately showed up a new wi-fi endpoint that was on the channel I was using. Needless to say, a simple channel change fixed my issue and normal service was resumed.