The speed of recording is determined by 3 things.
1 The disc's
2 The drive
3 The computer spec
With buffer under run you can let the computer do the thinking and just select the highest speed it will let you burn at.There are other factors to think about though.What your actually burning audio disc's or data disc's.Sometimes disc's burnt for audio won't play in some domestic/car stereo players if burnt at high speed.