For a start, flights to the USA leave from terminals 3 and 4 depending on airline.

Normal waiting time is less than 5 minutes. In fact you normally spend more time actually being looked at than waiting to be. Which is how it should be.

Just one recent occasion, waiting time 10-15 minutes. That was unusual. But you never know how it's going to be.

At the gate, depends on the airline. In Terminal 3 (AA/UA) you go into a holding room. In Terminal 4 (BA), just wait in nearby seating. Again, no real delay.

The real hangup going to the US is the checking in queue, if you have not done OLCI or e-ticket check in. AA always seems over 30 minutes in Y (much less in F/J). BA maybe the same looking at the queues but I have always used one of the alternatives. I genuinely do not know why it takes AA so long to proces people, you are at the counter for ever.

Both T3 and T4 have gates that are a long walk from the initial security point (or the lounge).