The 124 has, due to Russian engine technology, to stabilise power for a while before departure
That's the polite version
The way it was put to me one wet cold morning at Shannon a few years ago was that they spent that long at the holding point making sure that each engine was working well enough to produce enough power to ensure that the beast was going to unstick before the end of the runway
Based on subsequent observation, that could be true.
After using more fuel for the pre take off check than a 737 would have used getting to Dublin, the exchange over the radio went something like:
ATC call, "xxx clear take off, call passing through 1500 Ft."
XXX, "clear take off, call passing through 1500 Ft."
It duly lumbered down the runway and into the air, leaving a huge mist cloud hanging over the runway, and it used what appeared to be an indordinate amount of runway before it eventually rotated, and it dissapeared into the murk very shortly afterwards.
A good while later, I thought, "Ahh, he's forgotten the 1500 Ft call", but stayed on channel for a while longer.
Eventually, the call "xxx passing 1500 Ft" came over the radio, and he was then passed over to the next controller.
It had taken 7 MINUTES for the thing to get to 1500 Ft, which makes the suspicion of marginal power more than a little likely!
The other task that's "interesting" is trying to push one off stand in the cargo area at Dublin. It's slightly uphill, but what makes it almost impossible is that it also requires a sharp turn, which with the geometry of the wheels and the number of them is not easy.
The push back bar on it's own is over 2 tonnes, and has to be loaded back into the aircraft after it's been pushed, there's no way they are leaving that bar on station, it's too darn big and expensive to have one at every possible stopping point, so they have a bar handling system to get it on board after the push and before all the engines are started.
One one occasion, it was going out loaded, and the Servisair tug (their largest) that tried to push it back failed miserably, so Aer Rianta sent out an urgent call for assistance, and FLS had to come over with one of their twin engine monsters to get it off stand and into a position where it was safe to allow it to start engines, as where it stopped in the push was blocking access to all the C pier stands, and to most of the stands in the 40's & 50's, which did not greatly impress ATC.