I'm from the school of 'any time in an airport not actually spent walking to the gate' is wasted time', I think I come at this from the perspective of managing my airport time quite tightly.

Having said that, and also having commuted that section of the M25 for around 20 years until I moved to California 5 years ago, Karfa's point 3 above is a real possibility to build into your risk assessment - unless things have improved beyond recognition.

When the M25 runs well it's a miracle of civil engineering, when it doesn't it is indeed the road to hell