TT (time trial) helmets are purpose-built for racing against the clock, with their distinctive teardrop shape and long tail designed to smooth airflow when you're in an aero tuck position. They can save you 15–30 watts at 40 km/h compared to a standard road helmet—a meaningful advantage in competition.
However, that aero benefit only works if you maintain a low, tucked position the entire time. If you sit up, look around, or move your head frequently, the tail actually creates more drag instead of reducing it. TT helmets also have minimal vents, making them extremely hot—fine for a 40-minute time trial, miserable for a three-hour group ride.
If you're doing time trials or triathlons, a TT helmet is a smart investment. For everyday road riding, fast group rides, or training, you're better off with an aero road helmet, which gives you some speed benefit while still being comfortable and practical.