According to DCRainmaker, AW's HR sensor is worst than most. He has some pretty good data-driven analysis of the a variety of optical sensors, and honestly, the Garmin sensors are pretty good, even for intervals. You can look deep into the data for misses compared to a chest strap, but those do not really affect your workouts much. And to me, the benefit of the wrist HR far outweighs the downsides. I am through wearing chest straps. I use my 735XT for intervals, and I have no problems at all. Even the true professional athletes I follow that critique wrist optical HR often back down when they acknowledge how they use the HR readings and that the lags do not affect their workout effectiveness.