Gmail has become unusably slow
The Gmail team is most definitely aware of this and a fix is in the works.  At SXSW this year there was a panel with four or five Gmail team members and this came up — they said it's related to the size of your inbox, and that it has a lot of Googlers complaining internally too, since many of them have the oldest/large accounts.
Also see http://techcrunch.com/2010/03/14/gmail-slow/, though it has basically the same info.