Long friends list or guild list can cause lag if you have a slow connection. My connection is pretty good so I never really noticed but several guild members complained so we went from 7265 to 3200 members(booting old accts) and now I notice it being peppier too.

It was explained to me that due to some strange coding the system reloads your guild list each time you change rooms. Downloading 7k names will take awhile if your on 3g or having bad wifi.