Originally Posted by xxxxx

The T-35 cutoff is not just there so that people are in the right place at the right time. It is to check that everyone is "ready to fly". At T5 there are zero transactions at the gate, they scan you boarding pass, you get on, that's it. If someone didn't have APIS data in their booking or incorrect data, this can lead to delays with boarding. Also, Visa checks are done landside at a FBD if you're with hold luggage or at a visa desk if you're hand baggage only. Once your visa is checked, an indicator is added to your booking to say that it has been checked. Passengers requiring visas for other destinations that are not part of the visa check are limited and so this is done at the gate. This again removes the transaction from the boarding process.

The theory was also that there are no seat changes airside, once you're checked in, that's your seat. I don't know how that airside customer service desks/lounges are handling this and may still be changing seats. Though I know that seat changes will not be done at the gate.

So you see, it isn't just so that people are physically on time for their flight, it is also to remove risk of a delayed flight by removing complexity from the boarding process.