The new version of the Falcon 9 v1.1 Full Thrust, or as SpaceX would like everyone to start calling it "Falcon 9", uses super cooled LOX and RP1 to improve the density of the fuel for better performance in the engines, but also the ability to store more fuel in the same size tanks.
However, this introduces a number of operational issues. For one, there has been a limited amount of LOX and RP1 available at the LC-40 launch pad. Now that LOX and RP1 needs to be cooled, which takes time.
How much time does it take to cool a load of LOX and RP1, enough for a single launch?
This has consequences to how far apart a hot fire (that requires a full tank load) can be from a real launch. It also means recycling after an abort might have time constraints.