An aero race bike is built for maximum speed on flat and rolling terrain. It features deep, wind-cheating tube shapes, an aggressive riding position, and a stiff frame that transfers every watt of power to the road. If you're doing criteriums, flat road races, or sprint finishes, this is exactly what you want.
However, "fastest" depends on where you ride. Aero bikes carry a weight penalty — they're typically heavier than climbing bikes — so on steep, long ascents, that extra weight can slow you down. They also tend to be less comfortable due to the stiff frame and low, stretched-out riding position, which can wear you out on long days or rough chip-seal roads.
An aero race bike makes the most sense if your riding is dominated by flat terrain, group sprints, or fast criterium-style efforts. If your routes are hilly or you're riding long gran fondos where comfort matters, an endurance or climbing bike may actually be faster overall for you.