The slowing of heart rate prior to movement onset has been presented as a marker of task related cognitive processing and linked with performance accuracy. Here we examined this event-related bradycardia and task performance as a function of task difficulty. Forty experienced golfers completed a series of golf putting conditions that manipulated task difficulty by varying target distance, target size, and surface contour. Performance was measured by the number of holed putts and finishing distance from the hole. Physiological activity was recorded throughout. Analyses confirmed that performance varied as a function of task difficulty, worsening with longer distances to target, smaller targets, and sloping paths to target. Task difficulty also impacted the cardiac response, including the rate of heart rate deceleration, change in heart rate, and heart rate at impact. These heart rate metrics were found to correlate with performance strongly, moderately, and weakly, respectively. In conclusion, heart rate deceleration in the moments preceding movement onset was affected by task difficulty. Features of this cardiac deceleration pattern were characteristic of successful performance. Our findings are discussed in terms of the role of cognitive and motor processes during the execution of complex motor skills.