Belfast flights have historically been treated differently to other UK domestic flights as a result of thankfully historical "troubles". You have no idea what the cause of the delay was, perhaps airport technology, perhaps the late arrival of security personnel at the gate, which cannot be laid at the door of the airport company.
So far as queues at departure are concerned, the biggest problem is going to be Joe Public, who often appear to be oblivious to the security requirements with regard to stuff like liquids, belts, shoes, electronics etc, even though these rules haven't changed substantially for many years. It doesn't take many obstreperous passengers, arguing with security officials about what does and what does not constitute a liquid, or why they have to remove belts to extend waiting time by 8 minutes. It doesn't matter how well prepared you are for the ordeal of security, it's usually "the other guy" - bit like the post office queue.
I'm not trying to defend the airport management, but I'm afraid this is just one of a number of frustrations that confront air passengers today, and have done really since the late 1970s.