Originally Posted by
MaxFlyer
I recently flew JFK-LHR-DUB with the transatlantic with AA. Under 2 hour connection to get from T3-T5. Flight landed 15 minutes early at LHR but even with a long walk to flight connections, waiting for the transfer bus to T5, going to immigration and security I still had time for breakfast in the CCR.
ZAG goes from T3 so this is a very easy same terminal transfer for OP - no bus needed, just security.
And of course in your case you only had immigration because DUB is within the UK/Ireland Common Travel Area - there is otherwise no immigration at LHR for US<>EU connections on one ticket.