LH-20A1S burns very slowly but shows high speed

[qanda]This thread is about the Lite-On LH-20A1S. Click here to see full specs[/qanda]When I burn a Single Layer DVD Nero states a speed of 20x or around 27MB/s. Initially the remaining time is given as 3-4 mins, as I would expect. But when the 3 mins is up it is only 10% complete and the remaining time jumps from 0 to around 30 mins. The speed continues to be listed as 20x 27MB/s

smartburn media info:
Disc Type = DVD-R (Single Layer)
Disc MID = 54 59 47 30 33 00 (TYG03.)
Disc TID = 00 00 00 00 00 00 (…)
Nominal Capacity = 4.30GB
Manufacturer Maybe = Taiyo Yuden Company Limited
SMART-BURN Speed Limit = 20.0X (Write)

Using nero 7.1, winxp pro.

I have updated to the latest official firmware but no change.

A Pioneer DVR-112D 18x Dual Layer DVD took about 4 mins when it was recently installed.

Any suggestions will be received gratefully.