I would say that the heat isnt getting transferred efficiently to your cooling unit, I dont know if it is a measure of being "fast". The way to determine where the problem is would involve taking temperatures at different levels of the cooling system. Take the temp of the chip, take the temp of the IHS, take the temp of the water block/heat sink. If you have a significant temperature drop between any of those locations then it is the connection between those two units which is causing the problem.The heatsinks are not at all getting heat soaked, the heat simply isn't being dissipated fast enough.
Again, I dont think it is correct to characterize the heat transfer in the terms of speed, but I think you mean the same thing -- efficiency. I would say that active cooling systems are working better because they "overcome" the inefficiencies in the CPU package (I wont mention the exact location of the inefficiency because I know that will strike a nerve here) by going into it and "taking the fight" to the enemy -- the chip surface. Admittedly, it would probably be more accurate to say that the active cooling processes "pull" or "attract" the heat out of the critical areas (the CPU). Additionally, I would surmise that extreme active cooling systems like LN2 open other avenues of cooling because they cool the area of the motherboard down significantly so they cool the CPU package from everywhere. So if there is an inefficient layer placed between the CPU and the top of the CPU, it is not as significant to a LN2 system.The greater the temperature difference between the core and the cooling apparatus, the faster the heat transfer which is why the extreme cooling solutions work so well.