Yes. There are some crappy USB chipsets that only deliver 12 MB/s, not even allowing to burn at 8x.
The bad thing: There is no way to solve this problem. Additional controller cards (at least the PCI ones, don't know about PCIe-versions) also are restricted to about 16 MB/s (NEC chipset based).
Could you please try to find out, what USB controller is used in your computer?
In addition: Insert a written DVD disc and run the burst rate test with Nero CD/DVD speed.
Also, if you have an external harddisk: Get hdtune and run the benchmark.
Please post the results of these tests.