Usually the transfer is much smoother. Arrived in AA from JFK to T5 two minutes early with an hour and 45 minutes connection. Transfers from B gates aren't operating at the moment, so there's the shlep to A. The buses were also not running at full capacity, so there was a massive line of people - we had to wait for two buses. All in all, deplaning to arriving at T3 was a full hours.

Then the security line in T3: pandemonium. No fast track open, and only three security stands for hundreds of people. Some crying because they had been standing there for ages and about to miss their flights... We got through in 45 minutes, ran, and were the last two people on board (about 8 minutes before scheduled departure).

MCT from T5 to T3 is not currently 90 minutes, despite what Heathrow says - give yourself more like 2.5 to 3 hours, and that's without a delay of more than 15 minutes.