On a recent transit through Heathrow (to Edinburgh) my wife and I watched from the non-UK line as all those using a UK Passport tried repeatedly at the passport scanners, only to fail and have to then queue for the same Immigration Officers as the non-EU passengers were. It has been some years since we last went through Heathrow and it was an eye-opener. LHR is now on our personal 'no fly list'. It was quite obvious on both our arrival and later return experience that the airport simply does not have the systems and staff to cope with the volume of travellers passing through.

I had a look but can't find any list of average connection/transit times comparing airports around the world. Perhaps someone else knows of one. The MCT (minimum connection time) that airlines use don't really tell you much about one airport vs another either.