The maxiumum speed you can burn at is determined by many factors:
- Your writer (duh )
- Your PC (combination of CPU power and memory)
- The source of the information you want to burn (CD-ROM or hard drive)
- Other programs you run while trying to burn
You can never burn faster than what your writer supports (unless you somehow can magically patch it and overclock your writer (for some HP writers this was possible)), though stating the obvious on this one, some people misinterpret the numbers on the box and expect to write at 40 speed (yes, it actually happens)
Your PC has to meet certain system requirements in order to run the programs needed to write your own CD. A fast CPU and lots of memory provides you with a lot of power to ensure a high data transfer rate, but is no guarantee that it will burn successfully at all times. Slower PCs are not able to handle the data as fast as they should and will create a bottleneck. (a chain is as strong as its weakest link, the water can only flow as fast as it can flow through it's smallest point)
If you want to copy an audio CD, you have to take into account that not all CD-ROM (or DVD-ROM for that matter) are as capable for Digital Audio Extraction as most readers and this might slow down the process of data flow to your writer. A writer can only burn as fast as it receives the data that needs to be burnt on the CDR. Simple data (regular CD-ROM) has a higher transfer rate and almost never causes a problem in itself when you have a 40+ speed CD-ROM player (copy protection schemes excepted).
A Hard disk has a higher transfer rate than any CD-ROM, thus the chance of failure when you want to burn data from your hard drive onto a CDR is smaller than when you want to burn data directly from another CD-ROM.
Other programs you are running consume CPU power and memory as well. When other programs consume too much CPU/Memory power, it will get in the way of data transfer to your writer and thus creating a buffer underrun (where the writer receives no more data to be written, if it would continue when it had enough data in its buffer again, the error created would be to big for any reader to deal with and so the program ejects the CDR (named: coaster); Buffer underrun technology is accurate enough to continue where it left off and leaving such a small error almost all readers can overlook this using normal error correction).
This has become a long story and I hope it is clear. It will help to compare the data that goes to your writer as water in a river and the writer as a power plant that needs the water to run. There are many points where the water can stall and then prevent the power plant to supply electricity.
What I can tell you is this, a 12 speed writer won't do you any good in a 486 with 32MB memory. You need at least a Pentium with MMX technology (166MMX or up) or compatible to be able to burn properly, I would even advise a Pentium II with 64 MB of memory at least before you even try to burn at a higher speed than 4x