The data to be recorded goes like this:
Source/HardDisk/CDROM -> RAM buffer in memory -> Buffer in writer -> Laser
The RAM buffer can be almost as big as you physical memory and was "invented" to avoid buffer underruns if the buffer in writer was too small for the speed.
Something might be performing slowly between RAM and writer, but that should happen also in Feurio!
Maybe what Nero is showing is not what it's really happening with that buffer: every time that the recoder's buffer gets empty, the writer will stop until more data is received. Does the writer really stop when that happens?
What's your writer?
Unfortunately, I don't think it's possible to configure the Ultrabuffer en Nero, or disable showing the buffers (that consumes a little of time processing).