UPDATE 09/27/14 12:00pm CST: Performance has been snappy for a full 24 hours, so we're closing this. All OK.
UPDATE 09/26/14 11:15am CST: Our operations support at Rackspace have identified the problem and have made some initial changes. Performance on the affected server is excellent again as I write this, but we're going to leave this status update on yellow until we're sure the problem doens't recur.
We're experiencing intermittent performance problems on one of our frontline webservers. We're working with Rackspace reps to try to identify the cause and address it.