I just had an FLR(M) appointment there (and approved!).
It was a Thursday and appointment at 11:00, so arrived at 10:30. It was packed! Standing room only - we were number 37. Lots of families with young children. If you are not a fan of being in a small-ish waiting area with screaming and crying children, then be forewarned - get a good nights sleep and bring a suitcase full of patience.
All said and done, we were out at almost 3:30pm - about 5 hours!! They had one question for us (we needed to clarify we were under the old rules - which they then confirmed) and told us it would be about 45 minutes. We asked if we could leave for some air and food and they allowed us to do so - just had to report back to reception when returning.
All the employees we spoke with said they were having a very busy day - maybe this wasn't the norm and your mileage may vary. Incidentally, all the employees there were super-friendly, courteous and simply genuine. Even the security people were friendly and nice - a far cry from the attitudes where I had my biometrics done in the states.
As previously suggested here, I would also recommend getting an appointment as early as possible.