To put things into perspective to give an idea the fastest one can transit T5 to get to usually T5B I did it at a run in Feb 2018 during the snows at LHR and made it through security and arrived at T5B in 2o mins from stepping into T5 until I arrived at the gate....flight was delayed and we spent 4 hours on the plane waiting to be de iced before departing at 130 am instead of 830 ish. Travelling with family and hand luggage and not wanting surprises I say 1.5 to 2 hours.