I tested it and from what i can tell, the beam is definitely faster. Took 40s flat to take out a chaos spawn. Fireball with +5% monster and barrage took 50s, without them it takes 60+, making beam more than 50% faster, on top of being way more heat efficient.
Beam is able to take out over half of a spawns HP in one filling of heat, while the fireball managed more ~20%.