On Arion I initially used 8mm braided rope.  I later put rubber hose over the rope to see if it would reduce friction but the parrels tended to tangle.  Then I switched, following Arne's example, to 50mm nylon webbing, the same as you might find in car seat belts (good for crash gybes, ha ha), and the reduction in friction was significant.  I would not use any other sort of parrel now.

My first batten parrels were 10mm rope. These tended to catch each others as I was to hoist the sail.  I have later only used webbing, between 15 and 25mm wide. I can't say what they were made of, polyester or nylon. I once polished a mast after some use, and that cut a good deal of the friction.

Arne

PS: The geometry of the sail can aid or prevent the sail in moving easily up and down. One friction factor is short batten parrels. If the mast is jammed at the aft end of the batten parrels, the sail will be reluctant to move.