I arrived at the Embassy at 07:50 for an 08:30 appointment, the lines had already built since there were earlier appointments than mine. There are three lines, left for Immigrant, middle for none immigrant and right is where the NI go to queue for security.
The immigrant applicants go to the front of the line at security so you won't join the huge line. The rules on what you can take in have now been relaxed so mobile phones and tablets are allowed and lots of people are taking them in. It helps to relieve the boredom. If yuo want to learn more check the Ambassadors video from the USEmbassyLondon you tube page. He explains it all there (it was released september 2014).
I was into the main area by 08:20 which is a huge waiting room with secure booths down one wall. Tea, coffee, food is available from the front.
It took about 80 minutes before I was called forward to bring up my paperwork and have fingerprints taken.
I then has an unexpected short wait of around 10 minutes before i was called for my interview, which was 4 or 5 quick questions and then I was done within 5 minutes.