Arriving in DUB is very fast and very rarely delays clearing immigration like the UK for many passengers not Irish of course. If it's not that it's baggage delays. Handlers could learn a thing or two by being shown how things are done at DUB in general. At some UK airports operational efficiently is very poor and well below DUB when it should be the same.
The BA move to T5 is purely to try steal market share from EI which are carrying much higher numbers from both DUB and BHD, some BA flights are ghost flights compared to EI.