I have had plenty of 8x burns that are closer to 8 minutes. If I have been informed right, nec burners (the 3500 at least) can detect in the middle of a burn if it is going badlly and slow the burn down. If it actually does what it is suposed to do, I would think that would be a good function. It's beter to have it burn a little slower than to have a bad burn. I would think that it might also have something to do with the particular write strategy for that disk. Even a 16x burn generally takes over 6 minutes. It doesn't write at that speed through the whole disk and depending on the write strategy/firmware/drive it may only be writing at that speed at the very end of the disk.
I think that thier is a hack that will cause nero to show the actual burn speed, not the selected speed (not farmiliar with it though). try searching the nero forum.