The queues for the ground floor ladies (and sometimes gents too) at the Royal Albert Hall can stretch yards down the corridor - most frustrating when there's only a twenty minute break. It's generally quicker to nip up to the first floor where there are more facilities for fewer customers, and often no queues at all. Just rows of empty stalls. The upstairs bars are a little fancier too.
First floor stairs and conveniences are pictured below; check the RAH website for similar diagrams for the other levels.