from your harddisk you can always burn at the fastes speed. but don't do it with audio, some audio cd players have problems with it, don't go higer than about 8 speed orso.
If you want to do 'on the fly' copies, it depends ont he disk to read and the cdreader. The cdreader must be fast enough. And the master must be clear of dust and scratches, otherwise you read and you can a slow read speed..... i can copy almost all disk on 10 speed on the fly with nero with a extra buffer (on hdd) of 20 mb. If there is a hard to read sector on the disk those 20 mb will it not go cause a buffer under run.
damn what do i have bad english.