Don't burn up a bunch if DVD-Rs just to check this. You can just put one of the type you want to test in the input bin, select the image or other files you want to burn, UNCHECK the print box, and CHECK the "test write" box. It will still analyze the media and just turn the laser on to read power so as to not write on the DVD-R, but otherwise simulate a real burn.
My last communications with Primera indicated that they had finally gotten a iMac G5 set up with Tiger and Viola! they experienced the same slow burn: 11 minutes for 4.3GB on 16x Taiyo-Yuden media, the preferred brand for Plextor drives. Since that burn should complete in about 5 minutes (they don't really burn the whole disk at 16x), that's more than double the time. The Primera Tech believes that they have some USB 2 / ATAPI Bridge / Plextor problem, maybe with just Tiger, maybe with G5's, or maybe in general.
imho, it's too bad Primera dropped Firewire for the data transfer; it is so much more reliable (especially on Macs) and the speed is more consistent across Firewire implementations. Even the cheesy $39 Firewire disc box I bought works just fine with both a hard drive I tried, and the DVD-RAM I took out of my old G4/500DP. Oh, well.
Ed