Using SIW, check if your computer supports USB 2 (look in Hardware -> Ports -> USB): if it doesn't then you'll need to move that file to your internal HDD before burning because the USB connection is too slow for DVD burning.
You could also try finding 8x blanks, preferably from Verbatim or Taiyo Yuden, since your burner doesn't seem to fully support those 16x blanks you're using.