If SiSIDE and Win XP device manager show the drive as UDMA2, there is most probably no problem with the Windows configuration. So no problem here.
About your media: I cannot say anything about that, as I used Verbatim +R/-R and Fuji +R so far, nothing else. Ricoh and TDK DVD- and +RW, too.
Image creation: I never payed much notice to this. I sometimes defrag my hard drives now (something I never did before for 2 or more years ). When Nero creates an image, be sure to select "DVD-5" or so in the lower right corner! I once created a "DVD-9" image I burned to a 4.38GB DVD and the result was unreadable on another drive, though BenQ and LG read it correctly. And Nero is awfully slow in creating the image file, IMO.
DVDs not being read: That sound like a real issue to me ... never had a single DVD (ROM,-R/W,+R/W) my LG could not read! Get some more DVDs (movies, burned ones from friends etc) and try them all, maybe with the disc surface scan of Nero CD/DVD Speed. Incompatibilities when writing are not unusual (though firmware updates should make things better), but reading problems are new to me. Must be a really crappy writer to produce discs like that!
DVD writing time: Much too long. At 8x a disc should be finished in 8-9 minutes... if your drive needs that long for the start and end of the disc, I recommend to get one high-quality media disc (Verbatim DVD+R 8x or 16x, MCC03/04 media codes) and do one write on that. If that one takes as long as those cheap media you have some hardware defect. Also a try with some +RW media would be an idea - and you dont lose anything with that as you can erase it and retry.
Conclusion: The long write time and the problems with reading certain known-good DVDs sound to me like a problem with the drive itself. Do some more testing, try to read some rented movie dvds or so. At least pressed DVD-ROMs should be read perfectly and with good transfer rates in Nero CD/DVD Speed, but DVD-+Rs should be as well.