A high-density altitude decreases helicopter performance.
Density altitude is pressure altitude corrected for non-standard temperature. As such density altitude is a key factor in performance. With a high-density altitude, it is if the aircraft is operating in an environment that would exist at a higher altitude in standard atmospheric conditions. If the temperature is higher than standard, there will be less air pressure for that specific location. With less air pressure, the density is less. As a result, less lift will be produced as density is a key factor in the production of lift. Conversely, lower-density altitude will increase performance. If the temperature is lower than standard, the density altitude will be lower than the pressure altitude for that area. If the air density is high (higher pressure), there will be a lower-density altitude. If the air density low (lower pressure), the density altitude will be high.
FAA AC-00-6B Aviation Weather pg. 5-16, 5-17
FAA-H-8083-21A – Helicopter Flying Handbook pg. 7-2
FAA-H-8083-25B Pilots Handbook of Aeronautical Knowledge pg. 8-7, 12-5
Principles of Helicopter Flight, 2nd Edition, pg. 248