These problems get ignored by those who prevent the building of new runways.
A shortage of runway capacity certainly doesn't help, but a shortage, or inefficient use, of gates probably also contributes. It can feel frustrating when the captain comes over the PA saying "we are all set to go, but ATC won't let us start engines for another ten minutes", but that is probably less expensive overall than running the engines whilst stopped on a taxiway for those ten minutes.
Other than vacating gates, is there any good reason why startup should not be delayed until there will be no more than a couple of aircraft in the queue when one reaches the runway?
Taking the idea a bit further, is there any good reason why shorthaul flights cannot be held on the ground at their departure points rather than spending time in a hold at their destination? It often seems as if the last 20 miles from MAN to LHR takes as long as the first 150.
And why not re-open the existing third runway at LHR to reduce queueing times? Or is it really needed as parking space?