If the DVD drive is IDE and setup as a slave on the same cable as the hard drive you're burning from, try setting it as a master on its own cable.
Have you defragged the hard drive, and is there at least 15% free space?
If your burning software has the option to 'prevent buffer underruns', enable it.
Does it allow you to set the burn speed to x8?
Do you get the same slow speed when burning DVDRs (not RWs)?
If you were using cheap x2 DVDRWs, that could explain the slow speed, but Verbatim x4 discs should be burning between x2 and x4.
Alternative freeware:
Free ISO Burner
ISO Image Burner
Free Easy CD DVD Burner