If you are copying on the fly then your burn speeds are unlikely to reach 12x without the buffer leaping up and down. Same applies to 8x if your reader drive can't read fast enough. To copy on the fly at 8x, your reader needs to be able to read at a constant 10x or better, and not many readers can do that from DVD-/+R media. Also, your reader and burner would need to be on separate IDE cables/ports.
If you are burning from your hard drive, you should be able to burn at 12x, assuming your hard drive has fast enough data transfer speed, and ideally the hard drive and burner should be on separate cables/ports, more so if the hard drive is a little slow reading.
If the problem is none of the above, then make sure DMA is enabled for all drives/IDE ports. If not, then the data transfer rate will be much much slower than it should be.