I have departed from RAK twice recently. In December I was waiting at immigration for around 45 mins. Last week I was through in 2 minutes. Both times I was through security within 5 mins. Last week there was also a customs check before security, where everyone was being stopped. The guy before has his bag checked. I was just asked how much cash I had.

The line for check in moved pretty fast for the fast track line, and hardly at all for economy. Fast track is not available for BA pax. You need a lounge invite from the desk.

The lounge is really nothing special - random selection of sad snacks, and then beer/wine/soft drinks (subject to what has been stocked). I would say 90 minutes is a good amount of time to aim for (leave town in a taxi ~ 2 hours before?).