I'm experiencing the same.
Nuprol green, fills in about 8 seconds and fires lovely.
Propane adapter, I get about 3 shots worth.
On another note, I've got a mk18 that i got in a bundle deal I've noticed the nozzle seems to be dragging on the bolt. The spring seems snappy as it starts its journey back perfectly, but then i hear it rubbing, almost like air is escaping from it.
Edit, on closer inspection it looks like an AG drop in set, so not sure why its rubbing. I think that when it gets low gas/cold, the bolt cycles but the nozzle doesn't. So maybe a 170% spring is needed?
Edit 2: Post on pg90 says the AG buffer is too big and drags, so maybe that first?