It depends on your hardware too.
1) For starters run Nero CD-DVD Speed test for reading and check the curve, CPU Usage and burst rate, might not be enough to sustain 12X.
2) If you can waste a blank, run same program to create test disc, and see the ressulting curve.
3) When you burn, leave the computer alone, or do only light work on it.
4) Defrag your hard drive.
5) Try burning ISO and not on-the-fly compilations.