Probably defragging the hard disk will improve results, but if you have a slow drive is very difficult to eliminate these buffer issues.
Something that further cause slowing is that the operative system and data you are writing on DVD are in the same HDD, so every time the operative system need to access the disk, this will cause slowing in the burning process.
The ideal configuration is to keep operative system and data to be burned in physically separate drives.
Can you install a SATA HDD in your computer? This should improve a lot performance