Not just the An-124
It's not just the An-124 which does this: although it has a completely different engine, I've experienced something similar when travelling as a passenger in the An-74T. Lined up on the runway threshold, it went up to what seemed like 70% of full power, and sat there for nearly three minutes, before brake release and further throttle up.
What was unusual was that in two later flights on the same aircraft that day, the delay was shorter: 2:45 on the threshold for the first flight of the day, then 2:10 for the next flight, and finally "only" 45 seconds by mid afternoon (by which time ambient temparatures were in the low 30s). Waiting for core tempatures to increase was what occurred to me as the reason.
It's an usual aircraft: high wing, with a turbofan mounted over wing.
If had been able to speak Russian, I'd probably be able to tell you more...
Shamrock