Thanks psychohk, one of my pet peeves is people that dribble on to the runway at 2-3 knots, aiming to transition into a rolling take-off instead of stopping on the runway. More often than not they get cleared for take-off during the line-up and then take another 30 seconds to get going.
The other issue causing delays is that aircraft taxy and hold well short of the holding point. It is simply not possible to be in a position to use the runway should you get the call and you're stationary 200M from the holding point.
One possible reason for aircraft stopping short of the holding point at 07R is that larger aircraft (e.g. B777-300) have to make a very tight turn off taxiway J in order to stop at the holding point at J1. It can be done, but causes main wheel tyre scrubbing on the 777. I don't know what the current 'policy' is, but in the past we were urged to keep turns as wide as possible to avoid excessive tyre wear. I know it's not always possible due to sequencing, but one way of avoiding the problem is to allow the larger aircraft to enter J1 off H rather than J.