Fastest? You probably mean which is more responsive with minimal lag. That will depend on carriers since they add bloatware to the devices. Since the G4 isn't out in our country yet, no American can tell you what they think, because we just don't know yet.
Maybe someone will chime in that has a LG G4 "World" phone and they can put it against a Sammy S6 "World" phone so that you can have an apples to apples comparison. Just wanted to give you an FYI that you don't want to compare a current G4 to an AT&T/Verizon/Sprint/T-Mobile G6.