Controlled by the ecu I'm afraid.
On the earlier ones you could get a bit of adjustment on the pedal assembly but it's all enclosed on the later models.
Mines the same on idle about 600 rpm but it vibrates like hell.
hmm...mins at about 750, pretty quiet at idle for what it is. Pretty sure these engines are supposed to idle at 750 -+50 rpm, so Banshee is probably right, there will be a reason for the sluggishness....
mines always idled at around 600 rpm regardless of what fuel its got in,bio or derv.
on my old 98 tdi I adjusted the stop on the pot on the pedal stop to bring it back up to 750 ish rpm at idle.
its all enclosed on the later ones so cant really access anything to adjust it.