Hi and welcome to our forum.
I have the drive here and is using memorex, arita and verbatim at 8X using nero 22.214.171.124.
What do the log in Nero say? do it state that it writes at 8X?
-Check if DMA is enabled.
-How is it connected? I have mine as primary master, so you may try to connect it differently.
-Do the buffer of the drive stay above 80% while writing? or do it drop below 80%?
My drive needs about 8,5mins for a full discs at 8X.