Very quick, very expedient.
I returned a car here, 45 minutes before my flight took off. The employees here were very quick, efficient, and friendly. My customer service agent, Mirjana, calmly and politely helped an irate customer ahead of me, and had me on my way in no time.
It was a little unclear, but this is actually across the road from the airport. Go straight, not into the paid lot, from Split, and the return area is in a small orange building on the left. The map's pin directed me all the way to the wrong side of the airport.
Again, great service.